Import - way to do partially?
 

I am starting a new forum and would like to bring over threads from just ONE of my existing sub-forums. The Impex import script only does full imports.

Can anyone think of a work around for this need?

Just backup your database and restore into a new datebase, remove all forums that you don't need.

Sometimes the simple ideas are Brilliant!

Now, will that mean I have 12,000 registered Users I don't want? I assume I need to keep the registrations of the ones who posted in these threads. How would I go about the task of keeping only the 50 old users and discarding 11,950 others?

Again simple: ;)

Delete all unwanted forums (this will also delete the threads within).
Rebuild Counters
AdminCP -> Prune / Move Users: select all users with postcount < 1 (be carefull not to delete your admin account)