I really don't know appies well. But, have seen foal pictures of near solid babies, then progressive pictures as they age showing more and more spots. Seems they can change just about yearly and some even seasonally, at least for awhile.
I don't know if the registry would help or not, if the horse you suspect might be yours has DNA on File, I don't know if the registry would do a comparison for you or not. You'll probably just have to wait for the last owner to help.