If you've already been approved for the line, you won't have to pay a deposit again.
AT&T has a 14 day return/exchange policy... as long as you're in that window you'll be good to go, and will only have to pay the price difference of the phones (if there is one).
If it's been over 14 days, that means you won't be eligible to exchange the phone at the subsidized price and will most likely have to pay full price for which ever device you want.
Depending on where you bough it from, there might be a restocking fee. Just FYI.
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