I don't know how much it would cost to retolex a mark III, but I know it can get costly retolexing a 4x12 cab. (I posted a thread asking the same question for one of my 4x12 guitar cabs). I think that if it is done professionally, it will enhance (or at least maintain) the value of the amp. As far as changing into a different shell...I think that it depends on what you are putting it into. If you put your mark III into a AAA flame maple cabinet, it's going to increase the value of the amp. Where as if you were to fit it into an old shoe box, it would probably decrease the value. :lol: Hope this was helpful.