If they are not appear in the UI, I think you will need to use the REST Api to create a detection rule - check Service detection rules API - POST a rule documentation
Did you rename the Process Group of A to B? Or did you adapt the composition of the Process Groups? To merge services, they should belong to the same Process Group (renaming is not enough).
If the Services and Process Groups are the same functionality, then indeed it should be possible to merge the Process Groups using the Process Group Detection in the Settings and after that it is done, to merge the Services.
I can confirm renaming was not enough (-; created a new detection rule. If I did not make any typo’s, after the restart of the app service DT will only have one pool!