...but some local testing has confirmed that it is indeed moderately straightforward to upgrade my site to Drupal 5.1. The real motivation here isn't so much the upgrade (although D5 does have some significant improvements over 4.7), but to also provide an excuse to clean up the damn mess around here. As my first Drupal experience, it was a huge amount to bite off and the 1/4 of my original plans that I managed to execute, I did pretty badly :P Nasty raw SQL queries, hacked modules, horrible use of templates, yadda yadda.
Now that I actually know what I'm doing, I've been dying to clean the place up... both simplifying the site structurally and bringing it closer to my vision. Since I don't have to deal with importing several user/permissions systems (Nuke, phpBB, aMember, Gallery 2, and Wordpress [yes, I was insane]), things will run so much smoother once all the extra crap is ripped out.
So... sometime in the next few weeks, I'll have finished re-creating the theme properly, got the site upgrade path established, and tweaked things enough that I'll be able to bring it down for a day or so and do the overhaul on the live site. It probably won't look a heck of a lot different, other than cleaning up a lot of awkward bits and pieces. Oh and I'm 99% decided that I'm going to keep all photos on Flickr going forward and interface with their API to pull pics onto here, so I'm not duplicating all my galleries in two places.
It's really felt like a messy house around here. Started to weigh on my psyche or something.

Nice.
I've been told that Drupal is a cast-iron bitch to deal with.