Yes, the short story is that it’s a mess. However, there is no scenario whereby the GUF would “remove” flexibility. The problem is that, as you wrote, the TA can no longer change the ticket (unless they liaise with BA) and BA often cannot see the fare rules. So, what typically happens is that you would need to ask the TA to store the fare construction and rules in the OSI field of the booking. BA would then be able to action the change. Messy, to say the least, but doable.