Migration from uncategorized to General went well! All posts migrated. though I did find myself getting confused sometimes by the way Discourse refreshes itself when moving 30 posts at a time… probably not the proper way, but worked well enough.
Still having trouble with one category that is inaccessible for some strange reason, even though all of the posts show up. 
Converting categories with ~15 posts to tags turned out to be not painful at all when combined with reset bump date. 
Disable did the trick! Great to have users choosing their categories, especially now that we’ve weeded it down to just a couple.
I’ll be interested to see if users choose to create more categories now, or perhaps turn to embracing tags instead. I still haven’t settled on the best method for restricting tags. Users (current 2+ trust) enjoy making up new tags, but it is really matter of clarifying our useful tagging system in a manner similar to how they are individually colored and displayed as sub-categories previously. Might split into an all new topic, but I’d love to add clearer descriptions and colors to our various tags / tag groups, much like… well… categories.