The copy command was designed specifically for this purpose. To use this command, you’ll need to run the add command to add additional storages, and then copy all or selected backups from one storage to another.
You can also use third-party programs (like rclone) to copy the entire storage from one cloud storage to another, but if you go this route you’ll have to copy all backups – you can’t specify to copy only a few desired backups. Moreover, on local and sftp storages Duplicacy stores chunk files in a hierarchical structure (much like what git does), but for most cloud storages all chunk files are stored under one directory. Therefore, if you use third-party programs between storages with different chunk structures, you’ll need to flatten or unflatten the chunk directory first.