I dont know if you've already swapped phones, but I did a similar deal, and swapped phones with someone, I Traded my DX for a Thunderbolt. and all was well, then the update came and the phone went to $hitt with all the reboots and connection issues.
When i called CSR they told me that Even though the phone was a couple of months old. they could not honor the 1 year manufacture warranty on the phone, because it was not originally assigned to my account. so i could not claim the warranty. That the only way i could go about getting a new phone was to have "Lost or Broken" my phone and pay my 100 deductible.
then I asked to talk to someone in technical support And i told them that I didnt aggree with their Policy but regardless I was still having technicall issues, so i still needed help,
I told him that not once did the CSR Rep at the Store mention to me or the guy that i was swapping phones with, that if we swapped phones we were effectively voiding our warranty.
how i thought that should be communicated cuz it might detter people from doing it in the 1st place. He had me do a reset and he tried to boost the signal strenght i was getting (what ever that means). since he couldnt get it fixed. he checked with his mngr and he got my new phone approved.
Sorry this turned in to a novel.
But Its something people should really consider if they are considering trading phones.