Previously this could be done with SDs: the new owner’s signature could be added to the list of signatures. This is to mitigate the risk emphasized by @bochaco, the previous owner would put the SD only when the receiver has added his/her signature’s to the list (in addition to sender’s one). There is a regression because we cannot do this anymore with MDs,
Yes, but new MDs do no solve this problem either.
In conclusion I would say that a new data structure is yet to define to manage ownership transfer correctly.