so i really want an iphone 4 but i got the 3gs i just got it a while ago hasnt been 30 days yet can i go to at&t store and trade it for an iphone 4? the 3gs was 83 bucks so will they charge me like 120 for the iphone 4?plz reply apple is gonna forget about the 3gs in a while n i dont wanna b stuk wit it 