I stand by my reply in @chelsea’s thread. tl;dr: my reward is down-pledging.
I also love the idea of introducing a visible congratulatory signal that reflects how very very far above the goal line you have achieved. I don’t think that it should ‘bank’ the safe days or otherwise change the incentives of the yellow brick road. But retro-ratcheting is a brave and future-oriented thing to do, and so should be recognised.
Aside: I also use HabitRPG, and it’s surprisingly nice to dress up the little avatar and to make sure that he doesn’t die. I don’t make use of most of their ‘game’ mechanics, just the task/habit check lists. It’s a different way of tricking yourself that will work for some people on some habits.