sbs 2003 transition pack question

I need to upgrade from SBS 2003 to standard Windows 2003 and Exchange. I currently use the server only for AD and Exchange. I purchased the Transition pack and media, but am concerned about an “in place” upgrade. I was looking for some feedback on issues I might expect when doing this “transition”.

Also, is the 75 user limit for SBS a “hard” limit? Will the 76th user not be able to log in?

Any feedback or advice would be greatly appreciated.