That's what I've been doing all along. I didn't want to format my hard drive every time Apple came out with a new seed build. Right now I have 10.2.8 on my main partition and 7B74 on a smaller "sand box" partition. I can switch between the two easily. I don't really understand why you want to keep Jag though. There is an upgrade option in the installer, although I haven't tried it given my setup. It's pretty easy to keep your settings though, even in a format and reinstall. Just copy your home folder to a DVD or some other drive. The only folders you need to grab from your Library are Preferences and Application Support. Grab those from both your home folder (/~/Library) and from the system Library (/Library) and you're set. This might be a problem for you though if you're stuck with backing up to CDs though since you probably have several CDs worth of data to backup.