I failed to check one teensy tiny thing. The old install was still running as 32 bit. Because, well, the processor it ran on, with its single core, was a 32 bit processor.
The new one? 64 bit. Should be running a 64 bit OS.
Also, Ubuntu doesn’t seem to like the upgrade very much. It’s working, but there’s random errors, and various bits seem to have failed.
…so tomorrow I’ll be upgrading the upgrade. Well, nuking the site from orbit (it’s the only way to be sure). On the plus side, it means I can yank the 1Tb drive that’s just sitting there at the bottom of the case.