I changed my horse from Hunter Jumper to Western seat this year he is so much more happy if I jump with him at least once a week
Sometimes, if they really liked the old discipline then they would show signs of depression. I know horses that changed disciplines and hated it and showed signs of depression and some loved the new discipline. And ShowJumping horses (I know) tried barrel racing and they were really depressed. I wouldn’t switch disciplines in my opinion, I would find a horse that already does barrel racing.
Hope this helps