- Its basically no different to the internal hard drive of the Switch, except you can remove it. The only thing that's stored on it is the game's files, as well as screen captures and videos you save.
- A game's save data is always saved to the internal storage, not the SD card. All you need to do is re-download any archived games.
If you have an existing SD card and wish to transfer the data, you'll need access to a computer with an SD card slot. Insert the old microSD card into its adapter (which should have come with the microSD card), and insert it into your computer's SD card slot. Copy the Nintendo folder to your desktop, and swap out the microSD card with the new one and add the Nintendo folder you copied to your desktop to the new microSD card. Once its finished, you're pretty much good to go with inserting it into your Nintendo Switch.
If you do not have an existing microSD card, you can insert your microSD card into the Nintendo Switch and use the transfer tool within the Switch's options to transfer game data to the microSD card.