i had my horse scoped in October of 2015 and he was a Grade 1. Six months later, he was a Grade 3. So it’s my understanding that it can progress. The way it was explained to me, is that a Grade 4 is harder to fix because the cartilage has begun to stiffen and set up in the “closed” position making it harder for the surgeon to pull it back. The surgeons want to get in there and pull it back before that cartilage starts to “set up/harden” (forgot the medical term, similar to ossify - but cartilage)
My horse had the procedure done in March 2016.