[Started this as a question, but had an epiphany and solved my own question while drafting, so now it's a FAQ]
For whatever reason, I remained on 2.5 all these years. Finally figured it was time to upgrade to 3.3 - however at the same time figured I might as well go 'portable'.
My music library remains unchanged and unmoved - so I just launched 3.3 portable, pointed it at my music library, and was off to the races. A few small hiccups along the way:
1) had to play with the views for a while to get the layout I was used to
2) lost my playlists! No problem, found them in \\my music\musicbee\main\playlists, and copied the one I wanted to the \\library\playlists folder of my portable installation
3) lost all my 'date added' info! No problem; this data resides in the MusicBeeLibrary.mbl file. Found the old one in \\my music\musicbee\main , and copied it over to the \\library folder of the portable installation (where I had renamed the existing one .old). Launched MB Portable and Date Added data was back. (thought this might break something, but so far so good)
(NOTE: my paths above all show a \main subfolder - for some reason my active MB files and folders were on that 'level'. Also had an .mbl file and a playlists folder one level up, but they were obsolete. Not sure why, maybe I moved something back in 2012).
Hope this is helpful for someone else. If nothing else, I can reference this when I inevitably forget what I did!
And - HUGE THANKS to Steven for creating (and actively maintaining!) MusicBee!