by telling them the truth, can they replace with AWS compatible iPhone? (since I moved from ATT to T-mobile and paid the ETF and it is unlocked officially).
Seems as if you are offering up too much information. You can still tell the truth without telling them everything.
For example when i exchanged my wifes phone i told them that we bought out iPhones in November (which we did) and i told them that my phone is always on 4G but hers is mostly on E ( i failed to tell them that i had my phone exchanged a week prior but that was just neglecting to offer up information and not lying). So they switched it out. Don't tell them you are exchanging it to get the t-mobile variant. you need to come up with a real reason why you need to exchange it.