Update docs on payment methods deprecation

guides/source/developers/payments/payment-methods.html.md

@@ -30,6 +30,10 @@ service providers][payment-service-providers] article.
 - `available_to_users`: Determines if the payment method is visible to users.
 - `available_to_admin`: Determines if the payment method is visible to
   administrators.
+- `type_before_removal`: Contains the previous real payment type, in case `type` has 
+  been removed after switching Payment Service Provider. Defaults to `nil`. For more 
+  information, see the [Payment service providers][payment-service-providers] 
+  article.
 
 [payment-method-base]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/payment_method.rb
 [payment-service-providers]: payment-service-providers.html

guides/source/developers/payments/payment-service-providers.html.md

@@ -66,3 +66,16 @@ You would need to extend or rewrite this class with your preferred PSP
 integration.
 
 [credit-card-base]: https://github.com/solidusio/solidus/blob/master/core/app/models/spree/payment_method/credit_card.rb
+
+### Switching payment service provider
+
+After switching payment service provider, there may be `Spree::PaymentMethod` 
+records referencing a `type` class that does not exist anymore. Trying to 
+retrieve these records through an `ActiveRecord` query raises a 
+`Spree::PaymentMethod::UnsupportedPaymentMethod` error.
+
+If you cannot delete these records, you can deactivate them running 
+`rake payment_method:deactivate_unsupported_payment_methods`. 
+This way, their `type` will be set to `Spree::PaymentMethod`, allowing for 
+records retrieval without errors. Also, their real `type` value will be stored
+in the `type_before_removal` attribute.