With four retro tracks from the annals of Mario Kart history returning in Mario Kart 8, GameXplain has gone through the trouble of piecing together a head-to-head comparison video that pits the originals against their HD upgrades.
Mario Circuit, from Mario Kart Super Circuit, has received the biggest graphic overall since its GBA days, but it has also seen a slight modification to one of its classic turns. The other three tracks (Tick-Tock Clock, Grumble Volcano, and DK Jungle) all definitely benefit from the jump to HD, but there transformations seem to be mainly aesthetic at this point.
Mario Kart 8 is slated to hit the Wii U this May.
Mario Kart 8 - Graphics Head-to-Head Comparison [GameXplain]