I really don't think this will pan out. I managed to exchange one size a few days ago and to do a straight exchange they have to scan the original box and then the new replacement one. When the values are equal, it's a wash and you're good to go. Now, when the UPC is different and they go to scan, I'm sure there is going to be a discrepancy in price and there is a high chance they won't make the swap.
I didn't want to risk it, so went to stock.ga and put in the UPC codes for the sizes I wanted at Walmart. Found the limited inventory and went to two locations this morning to swap out 9 boxes. People in similar situations might want to do the same.