Hello Shella,
There are two types of SC which can be created in terms of services (i.e., planned and unplanned services).
Planned services are those whose nature and scope is defined at the time of procurement whereas in case of unplanned services the nature is not known at the time of procurement and it is known only once the service is performed.
Unplanned services also called as limits are restricted by a value limit or might be used in scenarios where the services are deemed to be trivial in nature of the services are provided for over a longer period of time.
Regarding your first question of classic and extended classic. Why do you want to change from Extended Classic to Classic at this point of time (i.e., after implementation and after in use by the client). Yes it should be possible to change but i dont understand the reason behind the change. Regarding the changes in process in classic scenario (i.e., for GR and SES), the way in which it is created would be the same as extended classic. You can do a GR in the backend or create a confirmation in the SRM system which would post a material document in the backend system.
Finally coming to your case of Service in the limit SC
While during the creation of PO
If the "GR based invoice" flag in vendor master is checked then the PO has item category D and you can enter a SES for it.
If the "GR based invoice" flag is not checked then the PO had item catgory B.
Can you investigate with the along options. This is as per the standard behaviour in SRM.
Additionally can you just post a screenshot of what exactly do you mean by the Service entered in PO.
Regards
Pramod