Here is the problem: the prefix is created properly, it shows up on the posts, works properly as a filter when I click on it...but if I do an advanced "Threads & Posts" search using a known good search term (which exists in the post text), and limit my search to that particular prefix, the matching posts do not show up.
So am I correct in thinking that once I am done assigning prefixes in the database (vs. editing the thread directly), that I will need to reindex the search engine? Luckily I can reindex during off-peak hours.
Still sorting the trainwreck of node, user and usergroup permissions, but that is due more to vB's clumsy system and the mess it created. Have spent a couple of hours on it already (with a lot of careful testing). May have questions--will post separately if so. Just one of the casualties of convering a forum with nearly 8 million posts.