It depends. First, you need to be a VERY good actor. You need to be a good mix of assertive and naive. It's hard to do. I have done it, but there are waysto tell that a shoe was/wasn't purchased from a certain store. Many inventories, however, overlap. For example, the sku #'s on a CDP could determinethat it was purchased at FinishLine. Take it to a Champs, and they'll know that, and the product will be unmistakably not theirs. If they scan it though,they still COULD accept it as a return, they just don't have to. You're best bet is to get lucky with a cool manager who doesn't really care. Youwon't be able to get out with a gift card or anything like that. You'd be lucky to exchange it for other products. But if you're just changingsizes, they probably won't care very much. Give it a shot.
Just say, "My grandma got me this as a gift (make sure you don't say "birthday gift" unless it's close to your birthday; they scan yourdrivers' license). She got the wrong size. She said if it didn't fit, that the manager at the Champs that she got it at said it could be exchanged atany other Champs." If they start giving you +$$%, turn up the aggressiveness. It is really hard to do though, so just be mentally prepared.