Seems to have worked, so far. The "Similar Posts" plugin borked, so I had to disable that. Everything else Just Seems To Workâ„¢.
So, what's the difference between Categories and Tags now? I used to use Categories as tags. Are they both interfaces to the same table in the database? One way to find out...
Update: oh. they're separate things. well, that's silly. so, now all of my old /tag/tagname links that used to point to category pages now point to empty "tag" pages because none of my posts have actual tags. That'll be fun to clean up...
Update 2: wasn't so bad. just went into the permalink settings and had "tags" point to /tags and left my "categories" pointing at /tag. Not exactly what they had in mind, but whatever.