Fix an issue with old payment methods from the original paypal extension.
user
```ruby
class RenamePaymentMethods < ActiveRecord::Migration
  def up
    execute <<-SQL
      update spree_payment_methods set type = 'Spree::Gateway::PayPalExpress' WHERE type = 'Spree::BillingIntegration::PaypalExpress'
    SQL
  end

  def down
    execute <<-SQL
      update spree_payment_methods set type = 'Spree::BillingIntegration::PaypalExpress' WHERE type = 'Spree::Gateway::PayPalExpress'
    SQL
  end
end
```
assistant