Hey @aGruber, thanks for reaching out to the Community!
I moved your post and started a new topic for you so that there is no confusion.
If they've already subscribed to Premium with Spotify, their billing will switch automatically and you won’t need to cancel anything. Your new payment date will likely be a few days earlier than usual to cover the price difference between Premium Individual and Premium Duo.
There is some more information on Premium Duo in this Spotify Answer.
I hope this helps. I'll be here if you have any further questions!