I think for Ultimate the XP rollover limit will indeed be 900 UXP, so maybe the new rule is that you can only rollover double than the current qualification threshold?

It would be strange to say the least because it would definitely discourage flyers from flying with AFKL. It could come as a liberation in a way to simply become a free agent after you get your 300 XP annually, but it would make little business sense for AFKL unless they have some tricks up their sleeve that will be announced soon as well.

It would also discourage upsells such as CO2 contributions or changing a flight to European business class for the XP, so will be very interesting to understand the logic behind. Unless they introduce something new at 600 XP.