thanks for posting, i hope you don't mind me replying in English !
Spotify does not offer yearly subscriptions at the moment, all payment are monthly based.
If you upgrade to Premium for Family your current Premium month will end as planned and as written on your account overview at Spotify.com and then and only then you'll be charged according to the new subscription.
In order to learn more on how to set up your Family plan correctly, please make sure to check out this help page.
Hope this helps 🙂
OneByBooRock Star 16
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