How can a Service Provider migrate customer from one backup storage to another backup storage, such as from Acronis Hosted Storage to Service Provider Hosted Storage?

There isn't an effective way to extract the backup data and migrate from one storage to another. The best way currently is to create a new account for customer in Acronis Backup Cloud, deploy them with the new account that points to SP Hosted Storage. And slowly let the old account (Acronis Hosted Storage) phase off due to the backup retention.