migration from exchange 5.5

i am running exhange 5.5 on an nt4 box service pack 6a. The network has a 2003 member server, acting as a pdc. I want to upgrade the nt4 box to 2003, leaving exchange 5.5 on it for the moment. Can i just put the 2003 disk in & upgrade the server & will echange carry on working. Also are there any issues i need to be aware of.