I have recently gotten into speed running, and have never realized that loading times (especially on the switch) in digital versions of games are much faster than SD and cartridge. I would like to start running this game, but I am wondering: if I were to get the digital version of the game, is it possible to transfer my save data from my cartridge version of MK8DX to the digital version if I were to buy it from the eShop, and if so how? I don't want to have to go through all the Grand Prix cups to get back all my kart combo's and star rankings. I know you could do something of similar fashion on the 3ds, but I want to make sure there is a way to do it on the switch before I go out and spend another $60 to buy the game again. Any help at all is valuable.

Don't quote me on this but im pretty sure save data is already saved on system memory/SD Card memory. So picking up the digital version of the game it will just use that pre existing save data.
While typing this i found a reddit post that says the same thing.
https://www.reddit.com/r/NintendoSwitch/comments/6wxkrq/if_i_buy_a_digital_game_that_i_already_have_the/

Well that is incredibly convenient, thank you, Slane

I remember back from MK8 on Wii U that switching from disk to digital kept my save files. I would assume that would be the same for the Switch too.

Yeah, Switch save files are stored on the system, so it should work for both.

It should, I did something similar for Mario Kart 8 (Wii U) and Xenoblade Chronicles 2 and it kept my save files. Save files are stored on the system and not on the disc/cartridge.

yea