I was wondering if anyone could tell me if this is possible, or if I would have to perform a complete re-install of the system?

Professional to Server is not a valid upgrade path as the two products are aimed at totally different markets.
Wipe and reinstall time - Why run Server on a workstation?

I have been sharing an application on my network to a few users, and those few users have turned into a few more users and now I am running out of available connections to the workstation. I can reinstall, but it would have saved me much work if I could have just upgraded the damn thing. Oh well. Thanks for the input!

Cheers!

What types of services are your users using?

There are also shady ways of turning your Pro into a Server.

Well, I only really need them to be able to access a single application across the network which runs from this single workstation. The application is designed to be run for "unlimited users" from a single centralized computer which has all the data on it.

In retrospect, I should've just bought server to begin with, (I put a SCSI hard drive in it!) but I was hesitant to believe that its use would catch on...

Any ideas would be appreciated!

look for a program called ntswitch it will allow more connections to pro without having to install server. However this is a potentialy dangerous way to do it. There is the possibility that you will hose the install. As with all hacks your experience may vary.

Beware ntswitch
Some aps will not install on the server claiming that they do not know this os, even if you 'ntswitch back' to workstation 8)
Anyway, it is a licence break and a lousy hack

yea it is a crappy option but an option none the less
I have never used it
I am running .NET enterprise edition here so I have no limitations anyhow.