The general rule with AS is anything goes. Pain can happen overnight or progressively, and it can stop overnight or progressively. You can have severe AS for years, then one day it goes mild. The opposite is also true.
There is no rule, no way to predict damage or disease course. Some have mild pain and a lot of fusion, some have no fusion but severe pain.
The only thing we can do is live in the now and try to live our lives like we have no disease when possible. If we have a bad time we go see our rheumy and try to find a treatment that works.