Well, after several abortive attempts and a great deal of coaxing, I was able cram the new drive into the TiVo successfully. I even have a backup of the old system and settings, so I can always bring it back later. It turned out to take me a bit longer than I expected, because the old drive would randomly die after extended use.
But after the dust cleared (literally... it was seriously dirty in that box after 3 and a half years use!) the new box is up and running with a whopping 198 hours and 50 minutes of space. Sweet!
Now off to do some chores, start packing stuff up for the long weekend at Pantheacon.