As far as I understand, you can SS or SS+ right up until you hit a BMI of 25, and then you need to move up to 810, because you'll likely be losing more lean muscle rather than fat. People do it differently, some people SS or SS+ it right until they hit goal, and then move up the stages a couple of weeks at a time. Some people when they're a stone away from their goal start moving up the plans. It's completely up to you really. You just have to move up to 810 once you hit a BMI of 25.
Every 12 weeks you have to have an 'add a meal week' which is a week of SS+. But after that, you're free to go back down to SS if that's what you want. Hope that helps!