Can you un-jailbreak a iphone?

I want to know, because i don’t feel like waiting for the next update. But i will only do it if you can undo the jailbreak. So when the update comes…...Wham! Back to norm and ready for the update. >>>Side qusetion; what happens after your phone gets locked out if you jailbreak and at&t locks it? Are you screwed? Do you have to pay for it to get it unlocked?