Even simpler than this, you can just eject the DLC virtual game card from the virtual game card menu without deleting and redownloading anything.
To be clear for everyone else, this is only necessary if you have the physical game and purchased the Anniversary Upgrade for the Switch 1 previously (including if you got it from the normal eShop listing just now thinking that would upgrade you instead of the in-game eShop link). If you only have the physical base game in the first place or have it digitally in any form, the in-game upgrade should work correctly.
das hat gerade geklappt... jetzt lade ich 50 gb...