Assume that I create a topic which isn't self moderated and you make a post. (You may not want to post in my thread if it's self-moderated). Then I make the topic self-moderated and delete your post.
I'm kinda on the fence about this idea; part of me agrees with OP that it'd be nice to switch a thread you created to a self-moderated one in order to get rid of shitposts (which has plagued so many threads on this forum, it's ridiculous). On the other hand, I suspect that if Theymos were to implement a feature like this, it would be the focal point of a massive number of complaints by members who had their posts nuked, even if said feature wasn't being abused.
A few years back, there was discussion about senior members making their threads self-moderated in order to combat low-value posts, but that seemed to go nowhere. I was one of the members who'd intended to do it, but I kept forgetting and in the end just gave up on it. So I guess the answer is to either make your thread a self-modded one from the get-go or take whatever is coming in terms of post quality. I don't think Theymos is going to change how threads are structured retroactively.