Last weekend I completed a Cisco UCCX 12.5(1)SU1 to SU3 in place ugprade. The reason for doing the upgrade was the myriad of bugfixes that come with SU3.
Specifically the following:
CSCwb92416 – UCCX 12.5: Memory dump causing high CPU and service crash
CSCvx74769 – UCCX 12.5SU1: socket I/O SSL memory leak
CSCwa79246 – Cloud Conect dataconn service should be removed as hybrid analyser is no more a supported feature -> Dataconn docker container is removed in SU3, so if you’re not using Cloud Connect (which we are not) it’s no longer chewing up memory.

Upon completing to upgrade and performing the “switch version” cli command:
admin: utils system switch-version
If you are switching to an earlier release, you must run:
utils dbreplication reset all from the publisher after all the nodes are switched over.
Modified configurations will not be migrated in case you are
downgrading this server to an earlier release level
Do you really want to switch between versions ?
Enter (yes/no)? yes
Switching Version and Restarting the Applicance ...
Switch version duration can vary depending on the database size
and platform configuration. Please continue to monitor
the switchover process from here.
Waiting ................................
Operation failed
ERROR: Acquiring lock failed

Ok, so we got “Acquiring lock failed”, in the end I rebooted the primary (the first one to be upgraded) waited for all the services to start then did the switch version, and viola, we were in business.
The same thing happened for the secondary as well. I hope this helps anyone else in 2023 upgrading the newest version of Cisco UCCX, these types of errors still occur!