I havn't done this myself yet, but I think you should be able to achieve your goal following these steps:
- downgrade the Essentials CPU license keys to v5 im the MyVMware portal
- set the new vCenter Server to Evaluation Mode (may actually not be necessary)
- add the v5 Essentials licenses to the vCenter 6 license inventory
- add the ESXi 5.5 hosts to the new vCenter Server and assign the Essentials licenses to the hosts
- re-assign the previously removed license to vCenter Server
André