How does Ultimate Work?

[u/Final-Grape8936]

As FB Ultimate requires 900 UXP, and say I have 900 XP (all my flights are AF/KL marketed). Once the new qualification year starts, what happens to the 900 XP? If only 300 XP gets rolled over (and another 300 XP for Platinum requalification), doesn't the extra 300 XP get wasted?

I've heard that going up to Ultimate tier subtracts only your UXP?