hi,
- Upgrade vcenter to 6
- shutdown the vms of one host
- rdmove the vms from inventory
- remove the host from vcenter
- connect the new esxi6 host and select the released license
- configure it if not already done (storage, network, ...)
- register the vms
- power them on
Next host.
Assuming you have shared storage, if not you'll probable need to use VMware converter and it will be much slower.
HOpe this helps.
Cheers.