Change a column type from Date to DateTime during ROR migration

Also, if you're using Rails 3 or newer you don't have to use the up and down methods. You can just use change:

class ChangeFormatInMyTable < ActiveRecord::Migration
  def change
    change_column :my_table, :my_column, :my_new_type
  end
end

First in your terminal:

rails g migration change_date_format_in_my_table

Then in your migration file:

For Rails >= 3.2:

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