The only thing that worked for me was placing a second password field somewhere before the actual one. The browser fill out the first and leave the second actual field empty.
It's of course not required and hidden by CSS and because I'm not processing it at all there won't be any problems.
I think it really should be supported to disable any auto completion on password fields where necessary, like password change like in your example.
I just came on this after implementing a user deletion option, which in my opinion is nuts when the required password field is prefilled. Same with password change option of course.