Ref T13222. See PHI683. Currently, you can "Change subtype..." via Conduit and the bulk editor, but not via the comment action stack or edit forms.
In PHI683 an install is doing this often enough that they'd like it to become a first-class action. I've generally been cautious about pushing this action to become a first-class action (there are some inevitable rough edges and I don't want to add too much complexity if there isn't a use case for it) but since we have evidence that users would find it useful and nothing has exploded yet, I'm comfortable taking another step forward.
Currently, EditEngine has this sort of weird setIsConduitOnly() method. This actually means more like "this doesn't show up on forms". Make it better align with that. In particular, a "conduit only" field can already show up in the bulk editor, which is goofy. Change this to setIsFormField() and convert/simplify existing callsites.