Create Changing Columns on migrations guides

[ci skip]

guides/source/migrations.md

@@ -415,6 +415,28 @@ end
 removes the `description` and `name` columns, creates a `part_number` string
 column and adds an index on it. Finally it renames the `upccode` column.
 
+### Changing Columns
+
+Like the `remove_column` and `add_column` Rails provides the `change_column`.
+
+```ruby
+change_column :products, :part_number, :text
+```
+
+This changes the column part_number on products table to be a text.
+
+Besides the `change_column`, the `change_column_null` and
+`change_column_default` are used specifically to change the null and default
+values of a column.
+
+```ruby
+change_column_null :products, :name, false
+change_column_default :products, :approved, false
+```
+
+This sets name on products to a NOT NULL column and the default value
+of approved as false.
+
 ### When Helpers aren't Enough
 
 If the helpers provided by Active Record aren't enough you can use the `execute`