That's exactly the point my friend. I had same success as you with high alk salt mix.... But then one day I decided to lower my nutrients. In my experience, lowering phosphate and nitrate in a tank that is established stresses corals unless u go very slow. When u have a high alk it is much less forgiving than if ur alk was 8. So when u add the ATS it's gonna lower nutrients, but prob way slower than what I did so maybe it won't be that bad.
I'm just trying to tell u what happened to me. I lost thousands of dollars of corals and don't want anyone else to go thru that. I will never keep high alk again.
As far as switching salt, I would start by goin half new salt to half current salt mix in your water change and slowly incorporate that til you switch over. CorAls are very adaptable but quick changes kill them.