class ChangePhoneToStringInSuppliers < ActiveRecord::Migration[5.1]
  def change
    change_column :suppliers, :phone_no_1, :string
    change_column :suppliers, :phone_no_2, :string
  end
end

This migration uses change_column, which is not automatically reversible.
To make the migration reversible you can either:
1. Define #up and #down methods in place of the #change method.
2. Use the #reversible method to define reversible behavior.