As I recall from my airline days, FQTV was the code we used when we added a frequent flyer membership number to a booking. Therefore if you add your number, it will show up on the airline's 'screen' as an FQTV. You won't actually see it on website bookings as website entries and responses are much more user friendly these days. If you want to change your frequent flyer programme on a BA booking, you'll need to delete the original and add the new one. The new number will then sit in the FQTV area of the airline's PNR. I don't know if you can change it on ba.com - maybe someone can help us out here.