Nothing you get to keep the promotion, phone returned to carrier for a restocking fee. If you are just exchanging then you are good and don't have to do anything.
If you are returning and buying a new one, basically you got a promo with the old one, paid restocking fee and you can submit another promo claim for a new one as long as you satisfy all the rules like buying within promotion period, uploading proof of purchase etc. Samsung isn't going to know or care that you returned your phone back and ask your promo back. They don't care for a few who misuse this and know majority of people aren't going to return. Probably they should develop more proper ways of verifying so that the return period is over and then check if the phone is still active in the name of the person submitting the claim etc. Probably too much hassle to develop such a system. Cost involved in developing that probably is a lot more than few hundreds or thousands free gifts for returned phones.
On the other hand if you call Samsung, may be they might say OK you can return the gift back to us or something like that. You can give it a shot.