hello @public1
Duplicati is a backup tool. It is not a migration tool. The best way to migrate is to use OS tools, that is, restore under the same system that has been used to backup, and then migrate the files to another computer with a different operating system.
The prompt "To proceed without losing remote data, delete all filesets and make sure the --no-auto-compact option is set, then run the backup again to re-use the existing data on the remote store.” does not say that it will allow you to keep your backups, but that it could allow you to keep your data.
The important difference is that the backup history allows you to know that a file has been deleted or created at some specific date, and to restore a given version. Keeping your data is much more limited; the backup will save the files at the date when you will backup, and the Duplicati deduplication will allow it to go faster, that is, see that data exists already on the backend and not send it again. But for a file that is currently deleted on your system but was existing in the past and has still data allocated on the backend, you would not get any way to restore it with this procedure. What you will get is ONE (1) fileset after this initial backup. The words ‘without losing data’ are a bit excessive IMO. It depends on what one is calling ‘data’.
To keep the backups (the history) would be a much more involved process - and is indeed not supported.