If you have a technical problem/defect with the item and You return it there is no restocking fee and you can try another phone and 30days will begin on date of purchase. I just got off the phone with tech support and made sure they noted on my account the problems i was having (cap upload, no mms pictures while on wifi, gps not locking). I specifically asked when I take back my epic back that the radio shack has no evo's in stock. If i was able to go into another store and purchase the evo and have 30 days to try it out. She said yes.
I had the same problem with the palm pre, it was defective. tried it out at walmart for 30days, took back because of technical problems i had with it and went to best buy a couple of weeks later once some of the fixes had came out and bought a pre. My upgrade was not affected, I do believe best buy did have to call sprint and verify but that was it.
I believe as long as you dont Start a new 30day trial within the 30day first trial purchase your fine... So if you got the epic the opening day aug31st. you'd have to wait till oct.1st to be able to have another 30day trial on a different phone.