I got a phone for my wife the other day a 3gs 4.2.1 firm with 6.15.00 BB... I tried to upgrade it to what i have on mine.. which is a custom 4.3.3 with 6.15.00.. well it frooooze to hell and had me screwed for a few hrs.. Finally got Redsnow to upgrade back to a stock( i believe) 4.2.1 with 6.15 with Cydia loaded!!! Seemed to work great on tmobile everything worked fine MMS data and all.. well she finally tried to unplug it and now it'll just power off when it wants to... I don't wanna jack it up again but would like to load the custom 4.3.3 that i have on my phone as i have had not one issue and all is good with it... Thanx in advance Sitting here waiting before she kills me!!

Do you have any SHSH on file for the new phone? Without them, you can only really restore iOS 5.0.1 at this point... Use TinyUmbrella to find out and let us know.