@Marc Harms | Otto's alles voor kantoor,Thanks for posting in Q&A.
From your description, I know that you want to downgrade MS365 premium to MS365 standard.
Based on my research, it is not feasible to downgrade from MS365 premium to MS365 standard directly, you should cancel MS365 premium subscription and purchase MS365 standard.
You can refer to following article to cancel your subscription, and you can also save your data following this link.
For purchasing MS365 standard, please refer to this link below.
Hope this can be helpful.
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