So I need to get an sd card with a bigger size so I can buy and download ultra sun from the eshop. If I change the sd card, would anything in my ultra moon or other games' save file be changed or deleted? The other games including the um is a cartridge not digital
As far as I know, no, nothing at all should happen to your games that are cartridges. The game data is saved to the cartridge and shouldn't have anything to do with the SD card.