Sorry, but some of these answers aren't quite right.
There isn't a rule about a BMI + 1 stone for anyone who's been doing the diet. That rule is for someone wanting to start Cambridge on SS.
For someone who has been doing the diet for a while, you must be on 810 or above once you reach BMI 25. You can then stay on 810 until goal. You don't have to move up the plans before that if you don't wish.