vCenter Server cannot manage newer host, so the first step would be to upgrade vCenter Server to version 7.0.
Then - with the 3 hosts maximum in mind - you may create a new cluster, and add one of the new hosts. Once things are properly configured (storage access, EVC mode, vMotion, ...), you may then start using the migration wizard to "Migrate both, compute and storage" to the new host.
Once one of the old hosts has been evacuated, remove it from vCenter, so that you can add the next new host, and continue migrating the VMs.
André