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If you are an existing Veeam Backup customer with Warm Backup Storage and want to change some of your storage to Cool Archive storage then you will need to create a new Veeam Backup subscription with Cool Archive Storage, backup to the new subscription and remove the old backups when you no longer require them. Unfortunately it is not possible to mass migrate from one kind of storage to another.
Contact our support team or your account manager if you need more information.