Thank you so much for your very thorough reply. Unfortunately, it made me realize that I hadn't been clear enough in the first place. So I'll try again:
We have a free TWA voucher for JFK-Cairo-JFK. It is compensation for a "bump" last March and is already ticketed. What strategy can I use, provided there is one, to get this changed to JFK-Cairo-CDG-JFK? Can I get it at no cost (given the voucher) or will I have to pay?
Thanks again for being so helpful. This list is great!