My mother in law is old and skittish about flying, and there is a really good chance that she will refuse to get on the plane for a rather expensive ticket. If I need to buy a Y, then so be it, but the price difference between Y and K is enough that I will ask:

On other airlines (e.g. UA), if a passenger cancels a non-refundable ticket, and then re-books a new ticket, the new ticket price difference is credited back to passenger in the form of a voucher that (IMPORTANT can be used by others as well.

Is this true on Delta? If mom-in-law refuses to get on plane, can I book a cheap no-show flight for her and then use the residual for my own use? Delta in India wasn't very useful on this point.