Tarrant has been discussed, Jezza, but just because Harvey was able to turn him into a good defender doesn't mean it will work in every instance.
As has already been mentioned, Roly tried turning Barlow into a half forward and we lost a good inside mid.
Kersten isn't working as a defender but we need him as a forward.
Apeness was drafted a tall forward and for all your Roly rahrah, 4 games at Peel as a backup ruck doesn't mean he will replace Sandi. Apeness is needed forward and if he doesn't turn out to be a great ruckman, he wouldn't be a wasted draft pick as you suggested, he will turn into another mismanaged player.
We need tall forwards. We don't need to be converting tall forwards into other things.
Oh, and Harvey succeeded with Tarrant. I haven't seen Roly succeed with changing any player yet but he's wasted a few.