He's not changing with my seat aid (yet). I have to do a strong half halt on the new outside rein and scoop with my seat. Chemaine explained it like this: It's being methodical with your aids, so the horse recognizes a canter transition when he hears one. Even if he's already cantering!
That way of thinking about it has helped a lot. The other thing she suggested was to keep cantering even if he doesn't change and just circle around and try it again. When he gives me one, we walk, and I give him a loud and enthusiastic good boy! He knows when he's done something right.
I am super excited to be schooling Third. Who would have thought?