Well, I just tried the 3.6 installer, and I must have done something wrong, because my BUG install is now totally borked. I had 3.51 installed, and didn't uninstall it first, so that's probably the main issue; but subsequently uninstalling both 3.51 and 3.6 and reinstalling 3.6 has left me with a still-borked install (and the backup Custom Assets folders are completely empty, which seems to me to be not what should happen with backup folders).
Here's the behavior: I start the game, reload a saved game I had in progress last night (before the 3.6 install), and it comes up completely UI-free. No control elements, either original or extra-crispy, are visible. Pressing Ctrl-Alt-O brings up a large window for BUG options; but that window is both modal and completely blank, with no way of dismissing it aside from popping out to Task Manager and killing CivIV itself (FWIW, this was also the exact behavior after the first 3.6 install, before I realized I had forgotten to uninstall 3.51 first).
I had already grumblingly resigned myself to the notion that all of my particular options were going to be wiped, and I was going to have to set up BUG to be exactly the way I wanted all over again; but I really don't want to have to do a wipe and reinstall of CivIV itself, if for no other reason than the fact that my game disks are put away somewhere "safe" (I use GameJackal so that I can keep my disks out of harm's way) and it usually takes me a couple of hour sof frantic looking to remember where I put them (also, I'd then have to reinstall the patches yet again, set up the game to start with GameJackal again, hide my master disks somewhere where the cats can't pee on them again, etc., etc).
