I can only say that when other parents describe ulceration, it seems to happen very fast. They don't have the time to write in and record the changes .... it is very sudden. Perhaps those that progress more slowly don't write in, though. I'll just say I'm ignorant on that subject and leave it at that.
I'd send a photo to Dr. Levitin and ask him. http://birthmark.org/node/152
You can send that photo to him and he usually gets back in touch quickly.