I really don't see it as a problem to back up your data and then do a reformat and install OS 9.0 then afterwards upgrade to 9.1. That is what I did and it really helped out a great deal. Of course other problems will crop up, but I have found with every major upgrade it tends to pay off in the end. As for reverting back, I am sure you would have to reinstall fresh as when you install back to 9.0, it won't over write what is more current. I am not really going there as I am not 100% and I would not tinker too much. I just like the reassurance that I either do it all or do not do it at all.