Can I upgrade to the 4.2 iOS and try jailbreak and then if it doesn't work revert?

As of right now, and the most current version of iTunes, if the SHSH blobs are saved, and by saved I mean on the local iTunes computer too--not just on Saurik's server--there should be no reason not to be able to downgrade. iTunes doesn't even know it is not talking to Apple when you redirect with hosts or tinyumbrella so if it can't be downgrade there is something else going on--not due to the blobs.

I have gone back and forth several times without issue. With both currently signed firmware as well as firmware no longer being signed. The key is the blobs. If they are saved, you are good to go... go back to older version that is.

Michael