A regular user can't, but theymos can and he is the only one that can do it. But only if there is a valid reason for it. This was discussed yesterday (I don't feel like searching for the thread) and someone shared a quote from theymos where he mentioned that he has the ability to make such conversions. However, only posts made after the conversion can be self-moderated by the OP. Posts written before aren't affected and can't be deleted.