Windows Server Backup has detected that the selected backup destination contains backups taken with a previous version of ...

Windows Server Backup has detected that the selected backup destination contains backups taken with a previous version of Windows. Performing a new backup to the destination will cause previous backups to get erased.
Copy the existing backups to a different location and run Modify Schedule to confirm that the backups can now be deleted, or reconfigure the backup schedule via Powershell or command-line by using the -allowDeleteOldBackups flag when setting the schedule.