A self-moderated topic has this warning on top:
This is a self-moderated topic. If you do not want to be moderated by the person who started this topic, create a new topic.
If you could change a thread into self-moderation later on, it wouldn't be fair for the users who don't want to be moderated by you.
An easy solution is to create a new thread, place a link in the old thread, and lock it.
This is right, and this is also a good a solution.
I think that's specially important in campaigns and bounties, because sometimes spammers just decide to spam in a certain topic for no reason, applying with multiple alts in a campaign.