-I know it's gonna be long and a pain in the ***, but do yourself a favor and do a clean install on these machines. Going from 9x to NT will do a number on the systems. It's better to do it now when you plan for it, rather than when you're in a cruch at a critcal time.
-I'd suggest taking a machine or two for testing purposes, and see how it flys for you. I can't guarantee all the software will work, so keep that in mind too. Check MS site to see if your apps will run under 2k. If they run under NT 4, 2k shouldn't be an issue.
-Make sure you have at least 128MB of RAM on all of them too, 64 will run--just barely.
-But first: Do your research on this. It may be better to just buy new systems than try to fiddle with them. Just look at all options first.
-Did I mention to have at least 128MB of RAM?
Good luck.;)