Thanks to everyone who helped out!

We successfully migrated the AIX 5.1 server (6H1) to AIX 5.2 using
alt_disk_migration cloning and then doing a migration upgrade install on
hdisk1 (while preserving AIX 5.1 on hdisk0). We had also done a bootable
mksysb to DVD-RAM beforehand with mkcd.

Was trivial -- only a few commands required from start to finish.

TSM 5.1 64-bit started up just fine under AIX 5.2 64-bit. Once we were
satisfied everything worked, after a while, we eventually removed the old
AIX 5.1 rootvg on hdisk0 and re-mirrored with AIX 5.2. Had there been a
serious problem with the AIX 5.2 environment, we'd have simply booted off
hdisk0 to gain the pre-upgrade AIX 5.1 environment and junked AIX 5.2 on
hdisk1. (This approach is similar to how you can use Solaris' flash
archives for OS installation on an alternate disk.)

As an unrelated note -- we had dsmc core dump with internal program error
while doing an incremental backup. Turns out it was because the Solaris
snapshot filesystem was corrupted internally with some bad metadata, so it
wasn't dsmc's fault after all. We unmounted, redid the snapshot, and
re-mounted it; dsmc ran fine that time. (We also figured out the cause of
the Veritas Volume Replication for the Oracle DB snapshot filesystem
corruption and applied a small adjustment to prevent it in the future.)

-Dan