I don't know how many times I've meant to create a self-modded thread and forgot to. It really would be nice to be able to switch, even if there were a time limit for a member to do so, e.g., you can't change a thread to self-moderated more than a day after it was started or whatever.
I made mistake like this and I agree that a window to switch topic mode from non-self-moderated to self-moderated is very helpful. Without it, I have to move mistakenly published topic to Archival, and create a new one.
In my experience, 20 seconds would be enough to realize I forgot to self-mod the thread.
I agree too. I realized my mistake after I published it. Within less than 20 seconds too but even 1 or 2 seconds, it is too late.