In the past when I switched ROM's for the G1 this is what you had to do: (should be the same but don't take my word on it till someone else conforms it since I'm not rooted)
1. boot into recovery and do a nandroid back up (gotta have this incase you mess up)
2. wipe/factory reset (cleans the phone so the new rom can go on the blank phone)
3. install the new ROM - same way you did before [that was the case on the G1] (wait for it to finish and it should return you to the recovery screen too)
4. reboot then, and your good to go!
5. reinstall your apps, if you have an app (forgot which ones but there are plenty out there) they can back them up so you don't have to go searching for them again.