The flyer is the one that keeps the frequent flyer miles even in bethel. So if you work in Legal or writting and you fly for branch business even though they pay for the ticket your the one that is keeping the flyer. So people just use their points or miles in order to upgrade when they want too. The branch or World Travel Desk will only pay for the economy class and will find the cheapest flight for all betheliets including GB members.
But not all travel is branch business. Assembly speakers is not paid for by the branch it is paid for by the circuit. Memorial speakers again not a branch trip but is paid for by the host congregation. Kingdom hall dedications again local congregation. So if a local congregation or someone who can afford to fly out a bethelite first class that is their decision to do it.