Although the computerized phone system was a bit like a maze, I eventually twisted my way to an actual person -no, wait. They were called .. "characters"?...
And after confirming my name and address, I asked my question and got a direct answer.
Just turn the invoice over, see the "return or exchange" section at the bottom, check the "exchange" box, fill in the new product number (found on the website), cut the bottom section off, tape the right side to the DVD being returned, use the left portion as the return label, et voila!
Less than 10 minutes later, the "full screen" version of The Lion, The Witch and the Wardrobe has been signed, sealed and ready for delivery to be replaced by the wide screen version.
And so I continue to be impressed with the Disney Movie Club.