My problem is not here, topic is no longer self-moderation when I brought it back to reputation.
Sure you can not revert it back to self-moderated.
It is similar to when you create a topic, if you want to make it as self-moderated, you must do it when you compose it and before you publish it. If you publish it as non-self-moderated (normal), you will not be able to convert it into self-moderated.
It is inconvenient but it has its meaning to prevent scammers to convert their topics from non-self-moderated to self-moderated when community discuss about their scam, show scam proofs, etc. I think it is reason why theymos controls this feature this way.