I just watched some footage of Mario Kart 8 from IGN and GameXplain, and as a long time Mario Kart fan, I am ecstatic that it basically confirms that all returning tracks will have an added twist to them. This is a huge step in the right direction for the series in my opinion, because sure there was some track tweaking in its predecessors, but nothing like this. This is just goddamn amazing in my opinion, and I'm going to pre-order my copy over the next few days thanks to the gameplay footage of both the old and new tracks (but for the sake of this entry I'm just going to talk about the old). Below are the currently seen retro tracks with their respective twists briefly explained alongside them.

Moo Moo Meadows (MK:Wii) - now has a brief optional gliding section.

Cheep Cheep Beach (MK:DS) - now has fully functional optional underwater segments to the sides.

Mario Circuit (MK:SS) - the hairpin turn is now a slanted anti-gravity section followed by a gliding section.

Toad's Turnpike (MK:64) - now has an optional anti-gravity sections where you can ride the outside walls, there are trucks that can be ramped off of to activate a gliding section.

Dry Dry Desert (MK:DD) - the bumpy section of the original track is now flooded, with the water flowing out of large holes in the ground (presumably bottomless pits).

Donut Plains 3 (SMK) - the track is now flooded, adding an optional and non-optional underwater segment.

Royal Raceway (MK:64) - the long jump is now a gliding section. So far it is the least changed track.

DK Jungle (MK:3DS) - the turns in the Golden Banana temple are a bit different, and it is now an anti gravity segment.

Rainbow Road (MK: 64) - now has several brief gliding sections with boost rings, heavily altered terrain for anti-gravity sections, and is a one-lap track with three checkpoints. The Chain Chomps have been replaced by Chomp Balls bouncing on undulating parts of the track. This is the most changed level so far.