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