It would be nice to have a post or two from Blizzard employees to see what went into this effort. I would assume most of Starcraft was written in C++, and that performing this kind of an upgrade without impacting other aspects could be non-trivial.
Why wouldn't they be able to pretty much just reuse all of the old engine code, if this is just a graphical update? Changing the graphics without changing the rest of the game should be fairly straightforward as long as it was written reasonably.
