if you don't have a reciept, most places will give you store credit or a giftcard instead of cash, but you can just turnaround and use that to buy the WideScreen version or exchange it straight across. Shouldn't be a problem. Try a place like Target or Kmart or something.
They're only located in the Midwest, but Meijer will let you return items without a receipt as well. The only problem is that you'll get stuck with a gift card equal to the value of the item instead of $$.