[ovirt-users] Migrating oVirt cluster from 4.0 to 4.1

We recently moved our oVirt cluster from 4.0 to 4.1 and have migrated all the hosts. The datacenter currently has a compatability setting of 4.1 but the cluster is still on 4.0. As cross-cluster migrations has been removed (no option anymore?) we aren't 100% sure how to get the cluster onto 4.1 whilst keeping the VMs online. Do we just change the compatibility version live and it "just works" or do we have to have downtime? Any info would be great! Regards, James