Thanks for the info Alistar, it's almost there. The bit that was stumping me was that after setting up a new dVswitch I was getting messages that the new dvSwitch was not reachable on the old host when I tried to migrate the VM.
The solution (courtesy of VMware Support) was that I have to add both dVswitches to one of the of the old hosts, then assign a spare physical uplink on that host to a temporary 3rd uplink on the new dVswitch. I then tag all the relevant VLAN's to that uplink.
That way I can first migrate the networking of the VM to the new dVswitch, then I can migrate the machine to the new cluster.
Obviously I can only migrate from old to new cluster with that one host. so each VM has to to do a double hop to the host with 2 switches, then to the new cluster. I also had to have both dVswitches at the same version.
Luckily I had a spare physical uplinks on the old hosts, otherwise I would have had to break one of the trunks and re use one of those uplinks.
Oh, and be aware, you can't use the same names for port groups on 2 DV switches, they have to be unique!
Nick