Let's say you plan on shutting down the old server. Since you want both exchange and DC on the new server, you'll need at least one more DC, even just a loaner, on the network.
1. DCpromo up the loaner. Transfer the FSMO roles here.
2. DCpromo down the current exchange.
3. Stop the IS and copy DB & logs somewhere you can get to them later.
4. Shut off the current exchange.
5. Create the new server as a member, same name, IP, etc.
6. DCpromo up the new server, transfer the FMSO roles here.
7. DCpromo down the loaner.
8. Install exchange with the /disasterrecovery switch on the new server. Put the logs and DBs you saved earlier in the correct locations. Put the permissions for the machine account on the org and server object using adsi edit. Start it up.