If you played The Witcher 3: Wild Hunt when it was new but not the Hearts of Stone and Blood and Wine expansions, and then other things came along and you kept putting it off until finally there was no realistic chance that you'd ever actually get around to it, but then CD Projekt released it for the Nintendo Switch and you thought to yourself, "I would love to complete this experience on that platform, perhaps in part while I ride the train to work, but I really don't feel like playing through the full base game again"—if you find yourself in those admittedly rather specific circumstances—then the latest "Switcher" update will make you very happy.
The Witcher 3 on the Switch is the Complete Edition, which includes both story expansions and "every piece of downloadable content released for the game." As of the 3.6 update, it will also be compatible with existing saves from Steam and GOG, "allowing gamers to continue their PC adventure on the go, as well as [with] HUD, menu and Gwent touch control support."
The Witcher 3 on Switch now has a "Cloud Saves" menu, as Kotaku explained, which will enable you to log into your appropriate account and then select the save you want to roll with. Do note, however, that "mod-related bugs and blockers" could be carried along with your save and cause headaches, and if you've changed the file name of your save for some reason, the Switch's Cloud Save feature won't be able to find it.

It's also apparently possible to go the other way, as the menu offers an option to upload your local Switch save to the Steam cloud. I've reached out to CD Projekt to clarify whether this means you can actually go back and forth from Steam to Switch—say, if you want to play on your Switch while you ride the bus to work and then continue on your PC at home—and will update if I receive a reply. (I can't imagine what the point would be otherwise, though.) In the meantime, you might want to make a backup of your local save before you start fiddling, to ensure you don't accidentally overwrite that old save.
Update: CD Projekt confirmed that the cross-platform saving works both ways, so it's not just a matter of exporting your PC save to the Switch: You can bounce back and forth between the two platforms as you like, carrying your game along with you.