Originally Posted by
hydrogen
In reality, you may encounter some resistance from the GA/other AC personnel who refuse to change your FFN/claim it cannot be done/claim that you will lose your J seat.
Thanks, I had a similar experience with UA agents who told me that I'd get kicked out of C if I switched my FF# too. I usually book my flights 6-7 days out so if there's no R when the SE window opens I probably won't even bother booking AC unless it's the cheapest.