The longer a horse is on the track the harder it can be to retrain. You're not starting with a fresh mind. The horse becomes set in it's ways and extra time needs to be spent working it out of it's resistance. They are used to living in a stall with a schedule. Few ground manners if any, hooked up to a cart and told to go like hell. Then back to the barn, a cool down, a bath then back in it's stall. Those are all the areas resistance will show up. It can be done as long as you are prepared to take the time to bring it along, sometimes by inches.