You CAN do it if you use QSM (QTree SnapMirror) instead of VSM (Volume SnapMirror). As an example, lets say you have a LUN in the root of a Flexvol on a system running 8.0, 7-Mode. You want to SM that LUN to a system running 7.3.3. You cannot use VSM since 7.3.3 does not know about the volume structure in a system running a higher OnTap release. In this case you would need to move your LUN to a Qtree (which can be done non-disruptively), and then you could use QSM to mirror the data from the OnTap 8, 7-Mode system to the 7.3.3 system. The reason for this is QSM is replicating only files (a LUN in this case) whereas VSM replicates the entire volume including metadata.