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That being said, if possible,we strongly advise you to first upgrade the target instance to at least the same version as the source. This would make the process much easier.
Can Configuration Manager for Jira migrate between Jira Cloud and Jira Server/Data Center and vice versa?
Configuration Manager is JIRA Server only and is not available for JIRA Cloud. The development architectures are completely different and CMJ's primary focus is Server/Data Center.
That said, a common way to handle this is to restore the Cloud instance on a blank Server/Data Center instance, and then use Configuration manager to handle the projects migration.