Lock the thread, move it to archival, start a new one that is self-moderated.
I highly doubt any mods will bother doing anything about your case
Mods don't change threads to self-moderated when there are already posts in those threads.
If you post in a self-moderated thread, there is a clear warning on top and you know OP can remove your posts.
If a thread would get self-moderated at a later point, early posters wouldn't know about this, hence the policy.