Well, the very nature of the Joker is that he changes; this was played on in TDK as the Joker came up with a different story for his scar every time he told it. The producers can use this as long as the change isn't completely off the wall. Just give the new Joker actor a different hair cut; different make-up applications; a different build (i.e. lost or gained weight while in Arkham); all of those changes are believable, realistic and can make someone look almost unrecognizable. Just look at the difference on Christian Bale as he lost all of that weight in "Rescue Dawn".