I thought you could change BA voucher matched tix once issued for a charge of $ 70 each, which is less than many US legacy carriers charge. Of course if you've already bought tix for travel beyond London, those tickets may be a problem.
Once you've booked the companion ticket, you can't change. At least that's what they told me over the phone and I'm pretty sure I read that as well.
I really shouldn't be whining as the trip will be great, and my daughter and I will enjoy the 1st class experience.
DD