Is it possible to have the technical reasons why this operation requires so much time please?
Because they don't use the automation tools that were originally built for Ryzom. They're configuring everything manually, and through hardcoded scripts.
Moving the game data is just moving a folder from the old Backup Service server to the new one. (Although, it's easier to just keep the BS on a separate cloud VM, it doesn't need to be on the dedicated game server box. That way you can just discard the dedi anytime.)
No idea about their proprietary web apps.
Last time I transfered my 5 shards to a new server it took about an hour tops.