It is not possible to switch a parent subscription for a domain subscription, it fails with an error:
An attempt to call external container for name servers failed due to unhandled errors during call.
Parent subscription is destroyed and does not exist in POA, but the reference to it was not cleared. The issue is considered as a bug, #PBA-52737, fixed in PBA 5.5.3.
Update to PBA 5.5.3 or later. If update installation is not possible at the moment, the only workaround is clearing the reference from the database. Please contact Parallels Technical Support for the assistance.