Apologies if this has already been covered, but I couldn't find it when I searched.

If I book a flight (for example LHR->DFW), and use the companion voucher, what happens if I need to extend my stay in Dallas? So, as an example:

- Book flight LHR->DFW on 1st March, returning 12th March, and use companion voucher. I'll just pay taxes & fees for the companion flight, I understand.

What happens if I need to stay in Dallas for an additional week, but my companion still goes back on the original date? Would there be any issues?

Thanks in advance, and Merry Christmas.