In Rails 3.2 and Rails 4, Benjamin's popular answer has a slightly different syntax.
First in your terminal:
$ rails g migration change_date_format_in_my_table
Then in your migration file:
class ChangeDateFormatInMyTable < ActiveRecord::Migration
def up
change_column :my_table, :my_column, :datetime
end
def down
change_column :my_table, :my_column, :date
end
end