Oh, okay. Then I’ll turn it into a UX issue. Although you are right that it says “retroactively” and the exact meaning of that is that old notifications will not be transferred, I did not interpret it like that. Why would one expect notifications that have already be sent out to be transferred to the new owner? Since only admins can change ownership, I see no risk for that someone would think so.
Anyway, how about changing the copy to something like
Note that the old owner will stop receiving notifications immediately and the new owner will receive all future notifications about this post.