@Ruegsegger, Jeff (Admin) Welcome to Microsoft Platform. Thank you for posting your query here.
Yes, You can change the retention policy for an existing restore point. As mentioned in document point number 4, you can switch policy or change the current policy.
Please refer to the document for the same: https://learn.microsoft.com/en-us/azure/backup/backup-azure-manage-vms#manage-backup-policy-for-a-vm
Hope this helps!
Kindly let us know if the above helps or you need further assistance on this issue.
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