You have 2 PBXs where one acts as a fail-over for the other.
You wish to move users easily onto this fail-over PBX within the portal without having to delete a re-add them against a second PBX service.
There is not, as yet, a simple solution since there are many potential scenarios to consider.
However, if your 2 PBX's are identical and the user SIP credentials match on both systems then, assuming the failover PBX exists on a different domain, all you need do is amend the domain in the PBX configuration page of the portal and re-save it. All users linked to the selected PBX will automatically re-register to the PBX on the new domain.