formato:
change_column(table_name, column_name, type, options): Changes the column to a different type using the same parameters as add_column.
Primeiro no seu terminal:
rails g migration change_numeric_field_in_my_table
Em seguida, no seu arquivo de migração:
class ChangeNumbericFieldInMyTable < ActiveRecord::Migration
def self.up
change_column :my_table, :revenue_per_transaction, :decimal, :precision => give whatever, :scale => give whatever
end
end
então
run rake db:migrate
Fonte:http://api.rubyonrails.org/classes/ActiveRecord/Migration.html