There was a thread on here last year (just searched my favourites can't find it) with details how to do it, it was written on Disney paper by a Cast member and scanned in by someone, I also have it printed off somewhere.
Recently there have been threads on Dibb saying a few have had success upgrading, I think I might have these links saved at work, will look tomorrow, if I have them I will post.
I believe most success has been in Hollywood studios guest services as you enter on the left, they seem to know how to do it there.
As PP says all who have managed it have said the start date of the AP is backdated to the start date of the ticket you have upgraded from.