Overall though, the install of RHEL over the top of RH9 was very easy - and a compelling argument for using logically different devices for /usr and /home (and a selection of others) even when they're on the same physical storage. It meant that the installer could easily just reformat the system filesystems and leave the ones containing user data untouched, leaving me with only a bit of reconfiguration to do post-install. I did, of course, have up-to-date backups anyway.
Now hopefully this machine will stay up for more than a week or so without kswapd grinding the system into the ground. I'd upgraded the kernel, watched as it happens, googled for information, and still have no clue as to what was going on. This wasn't the only reason for changing to the expensive version of Redhat, but I hope it'll be a benefit. At least I'll get to complain to somebody who's paid to listen if it does happen.
(Why not Debian or FreeBSD?)