I understand why someone would want to convert a regular thread in a self-moderated thread, but I see no point in doing it the other way around.
A member that do not want to make use of his Bitcointalk account anymore can want to make all his self-moderated topics to become moderated ones, such member can think his account can be compromised in a way his self-moderated can be manipulated by such bad actor.
More important, I think the most common situation is when people want to convert non-self-moderated topic to self-moderated.
Yes, and which is not possible than to create a new topic which is self-moderated.