Moving between namespaces is very easy and fast, as it just moves a few files with the snapshot index, owner, etc, as the chunks of the backup are in the same datastore. You can even do it from the filesystem if you feel confident to do so, but seldom recomend it as doing from the gui avoids human errors and doesn't need root permissions.
Moving between datastores requires copying both the index(es) and the chunks and can't be done from the filesystem.
In both cases to finish the "move", just remove the snapshot from the old datastore/namespace.