That's a great question. It seems to me that that Mark series heads typically sell for more than combos on ebay. If your combo shell looks really bad converting might help boost the value too. Of course, putting it in something cool like a hardwood/wicker shell will give it a significant boost.
That said, there is probably significant value in keeping the original combo shell of an older Mark I or Mark IIC+ from a collectors point of view. The safe route is to convert it and hang on to the combo shell. I've seen a few older Boogies for sale lately with both head and combo shells included. You can't go wrong with that plan.
