Como crio uma migração de trilhos para remover/alterar a precisão e escalar em decimal?

change_column(table_name, column_name, type, options): Changes the column to a different type using the same parameters as add_column.

rails g migration change_numeric_field_in_my_table

class ChangeNumbericFieldInMyTable < ActiveRecord::Migration
  def self.up
   change_column :my_table, :revenue_per_transaction, :decimal, :precision => give whatever, :scale => give whatever
  end
end

run rake db:migrate