We had a horse like this. We thought there might be some soreness or issue and had the vet out to do a full work-up. Nothing turned up. He was at such a high level in eventing that we expected that this “up” way of going probably worked for him and his last rider and became a habit. So, we JUST walked him for 8 weeks-up hills, down hills, at the beach, on the trails, asking for yields, shoulder-in, etc. We would throw all kinds of advanced riders on him to just have them walk around for a few minutes- 10 usually and get then just get off. (We thought multiple riders would be good since he had had a partnership with his last rider- and knew this one person.) We also walked him with other horses from the barn who were MUCH calmer. We always mixed up the ‘friend’ and also did it alone so no strong bonds formed that might cause a different issue. We allowed the reins to go to the buckle when the horse showed any signs of wanting to stretch and sometimes his nose would touch the ground. We had him in a snaffle and never pulled on his mouth if we could help it. He kept relaxing a bit more every week. Finally, around the eighth week, he seemed to understood he could relax with a rider on his back. It wasn’t perfect, but we saw intentional progress every week we kept it up. We still have walk-only days to reinforce his new lifestyle.