IPhone 3GS, new bootram, firmware 06.15.00, Current iOS 4.2.1

I have iPhone 3GS with following details: New bootram Modern firmware 06.15.00 iOS 4.2.1 SHSH blobs info: iPhone2,1 XXXXXXXXXXXX full blobs: 4.1, 4.3.3, 4.3.5, 5.0.1, 5.1.1 iPhone2,1 XXXXXXXXXXXX partial blobs: 4.2.1 Hi Team, I have jailbroken & unlocked my device earlier in 2010 and currently using without having any issues. After that i didn't upgraded my iOS. Right now i am planning to upgrade my iOS to 5.1.1 or 6(please suggest me which iOS has un-tethered jailbreak and unlock procedure) since may apps requires iOS 5 or higher. I searched many tutorials and finally wanted to verify my below queries. (provide links if any) If I upgrade my iOS do i need to jailbreak and unlock my iPhone? Since I have already had a jailbroken & unlocked device, is there any process to only upgrade iOS without jailbreak & unlock again? How to upgrade my device iOS from 4.2.1 to 5.1.1 or 6 (as per your suggestion) without changing my modern firmware with new bootram? If i upgraded my iOS to 5.1.1 or 6 do i need to activate my iPhone again with AT&T SIM? If I found some issues once I upgraded to iOS 5.1.1 or 6, can i revert back to iOS 4.2.1(as I had partial blobs available for 4.2.1)?