diff --git a/activestorage/lib/active_storage.rb b/activestorage/lib/active_storage.rb
index 5c5da551ae4fbc714c591ea67df4e211a8e94dc9..75eb63f0b404452ab31d836bd8c6de81ef512b8b 100644
--- a/activestorage/lib/active_storage.rb
+++ b/activestorage/lib/active_storage.rb
@@ -43,18 +43,26 @@ module ActiveStorage
 
   mattr_accessor :logger
   mattr_accessor :verifier
+  mattr_accessor :variant_processor, default: :mini_magick
+
   mattr_accessor :queues, default: {}
+
   mattr_accessor :previewers, default: []
-  mattr_accessor :analyzers, default: []
-  mattr_accessor :variant_processor, default: :mini_magick
+  mattr_accessor :analyzers,  default: []
+
   mattr_accessor :paths, default: {}
-  mattr_accessor :variable_content_types, default: []
+
+  mattr_accessor :variable_content_types,           default: []
+  mattr_accessor :binary_content_type,              default: "application/octet-stream"
   mattr_accessor :content_types_to_serve_as_binary, default: []
-  mattr_accessor :content_types_allowed_inline, default: []
-  mattr_accessor :binary_content_type, default: "application/octet-stream"
+  mattr_accessor :content_types_allowed_inline,     default: []
+
   mattr_accessor :service_urls_expire_in, default: 5.minutes
+
   mattr_accessor :routes_prefix, default: "/rails/active_storage"
 
+  mattr_accessor :replace_on_assign_to_many, default: false
+
   module Transformers
     extend ActiveSupport::Autoload
 
diff --git a/activestorage/lib/active_storage/attached/model.rb b/activestorage/lib/active_storage/attached/model.rb
index ae7f0685f23135578b0485355ffb2325db6644ac..06864a846fb71a9e90d4b5cff3a60a3778e835fd 100644
--- a/activestorage/lib/active_storage/attached/model.rb
+++ b/activestorage/lib/active_storage/attached/model.rb
@@ -93,12 +93,19 @@ def #{name}
           end
 
           def #{name}=(attachables)
-            attachment_changes["#{name}"] =
-              if attachables.nil? || Array(attachables).none?
-                ActiveStorage::Attached::Changes::DeleteMany.new("#{name}", self)
-              else
-                ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, attachables)
+            if ActiveStorage.replace_on_assign_to_many
+              attachment_changes["#{name}"] =
+                if attachables.nil? || Array(attachables).none?
+                  ActiveStorage::Attached::Changes::DeleteMany.new("#{name}", self)
+                else
+                  ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, attachables)
+                end
+            else
+              if !attachables.nil? || Array(attachables).any?
+                attachment_changes["#{name}"] =
+                  ActiveStorage::Attached::Changes::CreateMany.new("#{name}", self, #{name}.blobs + attachables)
               end
+            end
           end
         CODE
 
diff --git a/activestorage/lib/active_storage/engine.rb b/activestorage/lib/active_storage/engine.rb
index f70d0a512acaf0dcae94617b11d2aec4925a4220..e88e7fa14e9fc3d9bbcf404d318c53250c58d4a3 100644
--- a/activestorage/lib/active_storage/engine.rb
+++ b/activestorage/lib/active_storage/engine.rb
@@ -79,6 +79,8 @@ class Engine < Rails::Engine # :nodoc:
         ActiveStorage.service_urls_expire_in = app.config.active_storage.service_urls_expire_in || 5.minutes
         ActiveStorage.content_types_allowed_inline = app.config.active_storage.content_types_allowed_inline || []
         ActiveStorage.binary_content_type = app.config.active_storage.binary_content_type || "application/octet-stream"
+
+        ActiveStorage.replace_on_assign_to_many = app.config.active_storage.replace_on_assign_to_many || false
       end
     end
 
diff --git a/activestorage/test/models/attached/many_test.rb b/activestorage/test/models/attached/many_test.rb
index 878e284049f5e5c3ffc133fc3ae54aee3950c4e8..39ddecb0411302063058401d33abe92d27121d07 100644
--- a/activestorage/test/models/attached/many_test.rb
+++ b/activestorage/test/models/attached/many_test.rb
@@ -269,6 +269,24 @@ class ActiveStorage::ManyAttachedTest < ActiveSupport::TestCase
     end
   end
 
+  test "updating an existing record with attachments when appending on assign" do
+    append_on_assign do
+      @user.highlights.attach create_blob(filename: "funky.jpg"), create_blob(filename: "town.jpg")
+
+      assert_difference -> { @user.reload.highlights.count }, +2 do
+        @user.update! highlights: [ create_blob(filename: "whenever.jpg"), create_blob(filename: "wherever.jpg") ]
+      end
+
+      assert_no_difference -> { @user.reload.highlights.count } do
+        @user.update! highlights: [ ]
+      end
+
+      assert_no_difference -> { @user.reload.highlights.count } do
+        @user.update! highlights: nil
+      end
+    end
+  end
+
   test "attaching existing blobs to a new record" do
     User.new(name: "Jason").tap do |user|
       user.highlights.attach create_blob(filename: "funky.jpg"), create_blob(filename: "town.jpg")
@@ -538,4 +556,12 @@ def highlights
       User.remove_method :highlights
     end
   end
+
+  private
+    def append_on_assign
+      ActiveStorage.replace_on_assign_to_many, previous = false, ActiveStorage.replace_on_assign_to_many
+      yield
+    ensure
+      ActiveStorage.replace_on_assign_to_many = previous
+    end
 end
diff --git a/guides/source/6_0_release_notes.md b/guides/source/6_0_release_notes.md
index c1520766283c923ec91e51cec74683c53e6ce050..7c5478a03d4a7dd67dd515fa8ad679c98f2f6ca9 100644
--- a/guides/source/6_0_release_notes.md
+++ b/guides/source/6_0_release_notes.md
@@ -672,6 +672,12 @@ Please refer to the [Changelog][active-storage] for detailed changes.
     is saved instead of immediately.
     ([Pull Request](https://github.com/rails/rails/pull/33303))
 
+*   Optionally replace existing files instead of adding to them when assigning to
+    a collection of attachments (as in `@user.update!(images: [ … ])`). Use
+    `config.active_storage.replace_on_assign_to_many` to control this behavior.
+    ([Pull Request](https://github.com/rails/rails/pull/33303),
+     [Pull Request](https://github.com/rails/rails/pull/36716))
+
 *   Add the ability to reflect on defined attachments using the existing
     Active Record reflection mechanism.
     ([Pull Request](https://github.com/rails/rails/pull/33018))
@@ -688,10 +694,6 @@ Please refer to the [Changelog][active-storage] for detailed changes.
     `mini_magick` directly.
     ([Pull Request](https://github.com/rails/rails/pull/32471))
 
-*   Replace existing images instead of adding to them when updating an
-    attached model via `update` or `update!` with, say, `@user.update!(images: [ … ])`.
-    ([Pull Request](https://github.com/rails/rails/pull/33303))
-
 Active Model
 ------------
 
diff --git a/guides/source/configuring.md b/guides/source/configuring.md
index e53e8b4e926722bc6eac4e6142dd0d58bf5e78bb..4e0224b61e2100d8145057800133d3cd7207d072 100644
--- a/guides/source/configuring.md
+++ b/guides/source/configuring.md
@@ -881,7 +881,9 @@ text/javascript image/svg+xml application/postscript application/x-shockwave-fla
   config.active_storage.routes_prefix = '/files'
   ```
 
-  The default is `/rails/active_storage`
+  The default is `/rails/active_storage`.
+
+* `config.active_storage.replace_on_assign_to_many` determines whether assigning to a collection of attachments declared with `has_many_attached` replaces any existing attachments or appends to them. The default is `true`.
 
 ### Results of `load_defaults`
 
@@ -917,6 +919,7 @@ text/javascript image/svg+xml application/postscript application/x-shockwave-fla
 - `config.active_job.return_false_on_aborted_enqueue`: `true`
 - `config.active_storage.queues.analysis`: `:active_storage_analysis`
 - `config.active_storage.queues.purge`: `:active_storage_purge`
+- `config.active_storage.replace_on_assign_to_many`: `true`
 - `config.active_record.collection_cache_versioning`: `true`
 
 ### Configuring a Database
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index f17955e02210b27d138a5eced54aa8da3756336b..05980d1614ea9fd4e4e6e68567e5f2a96889f356 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -398,6 +398,54 @@ config.load_defaults "6.0"
 config.autoloader = :classic
 ```
 
+### Active Storage assignment behavior change
+
+In Rails 5.2, assigning to a collection of attachments declared with `has_many_attached` appended new files:
+
+```ruby
+class User < ApplicationRecord
+  has_many_attached :highlights
+end
+
+user.highlights.attach(filename: "funky.jpg", ...)
+user.higlights.count # => 1
+
+blob = ActiveStorage::Blob.create_after_upload!(filename: "town.jpg", ...)
+user.update!(highlights: [ blob ])
+
+user.highlights.count # => 2
+user.highlights.first.filename # => "funky.jpg"
+user.highlights.second.filename # => "town.jpg"
+```
+
+With the default configuration for Rails 6.0, assigning to a collection of attachments replaces existing files
+instead of appending to them. This matches Active Record behavior when assigning to a collection association:
+
+```ruby
+user.highlights.attach(filename: "funky.jpg", ...)
+user.highlights.count # => 1
+
+blob = ActiveStorage::Blob.create_after_upload!(filename: "town.jpg", ...)
+user.update!(highlights: [ blob ])
+
+user.highlights.count # => 1
+user.highlights.first.filename # => "town.jpg"
+```
+
+`#attach` can be used to add new attachments without removing the existing ones:
+
+```ruby
+blob = ActiveStorage::Blob.create_after_upload!(filename: "town.jpg", ...)
+user.highlights.attach(blob)
+
+user.highlights.count # => 2
+user.highlights.first.filename # => "funky.jpg"
+user.highlights.second.filename # => "town.jpg"
+```
+
+Opt in to the new default behavior by setting `config.active_storage.replace_on_assign_to_many` to `true`.
+The old behavior will be deprecated in Rails 6.1 and removed in a subsequent release.
+
 Upgrading from Rails 5.1 to Rails 5.2
 -------------------------------------
 
diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index 50d43ff69ed91e994b590b6243ebd968239a673e..934578e9f12c0a81afd62cc5c158ce4bff48af95 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/railties/lib/rails/application/configuration.rb
@@ -145,6 +145,8 @@ def load_defaults(target_version)
           if respond_to?(:active_storage)
             active_storage.queues.analysis = :active_storage_analysis
             active_storage.queues.purge    = :active_storage_purge
+
+            active_storage.replace_on_assign_to_many = true
           end
 
           if respond_to?(:active_record)
diff --git a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
index abb03e761bbe4632e12ceaee114ba0288657a6d7..ffe53497bf434b341e8e2cbad605cb709212c92a 100644
--- a/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
+++ b/railties/lib/rails/generators/rails/app/templates/config/initializers/new_framework_defaults_6_0.rb.tt
@@ -26,6 +26,10 @@
 # Rails.application.config.active_storage.queues.analysis = :active_storage_analysis
 # Rails.application.config.active_storage.queues.purge    = :active_storage_purge
 
+# When assigning to a collection of attachments declared via `has_many_attached`, replace existing
+# attachments instead of appending. Use #attach to add new attachments without replacing existing ones.
+# Rails.application.config.active_storage.replace_on_assign_to_many = true
+
 # Use ActionMailer::MailDeliveryJob for sending parameterized and normal mail.
 #
 # The default delivery jobs (ActionMailer::Parameterized::DeliveryJob, ActionMailer::DeliveryJob),