The problem is that there are a bunch of tables in which user IDs appear, and in some of them, they appear in multiple fields. I dealt with something similar in migrating from my SME server, and did the find/replace across all fields of all tables–see this thread for some discussion of tools available to do that:
The one I went with was the link in my OP to interconnect/it.
You’d obviously want to take a backup of your database first.