When I migrated the migration tool was not as mature and I complete all migrations manually, its actually much more simple that it seems.
In the manual process I needed to migrate volumes which were snapmirrored from a 7mode appliance to a 7mode appliance in another data center. The volume was large and creating a new snapmirror would have left it without protection for several days.
I snapmirrored the source volume to the new ontap9 appliance and snapmirrored the destinations volume to the new ontap9 appliance in the second data center. During after cut over I created the snapmirror relationship between the new ontap9 volumes the snapmirror was converted to new ontap9 snapmirror and resyncronnised without massive data transfer.
I believe this is similar to what you are trying to do, I think the tool can also do this.