Mario Kart DS started the tradition of including retro courses in each new release, but it wasn’t the first in the series to do so. Super Circuit has all of the original Super Mario Kart’s tracks as unlockables. It made sense, given the hardware limitations of the Game Boy Advance and the similar playstyle to the Super Nintendo entry.
These tracks were exact copies of their original design. Mario Kart DS’ recreations were also near identical. However, some courses in Mario Kart Wii were slightly altered to fit the game’s new trick system.
I never played Mario Kart 7, but 8 makes some tracks almost unrecognizable. The addition of low gravity sections, along with new underwater paths and jumps, made retro courses more like re-imaginings.
I’m split on which direction is better suited for the series. In some ways, it feels lazy to just slap old courses in. New elements aren’t being used to their full potential on these older tracks. But I can’t say I’ve ever played a redesigned course that felt like an improvement over a past version.