I googled on this and found an earlier thread here that I actually started. Was trying to migrate a vfiler "whole" to a new cluster, but from 7.3.5.1P% to 8.0.2 and with no snapmirror license. Ended up doing it via rsync to a new vfiler.
Looking into getting SnapMirror licensing now. And the target cluster is now 8.1 7-mode. To confirm, I can move a vfiler from 7.3.5.1P5 to 8.1 "whole" with just the minor interruptions during the cutover? Source and target are on the same subnet.