you need different license keys for version 6.
Best way to go about is, go to your my.vmware.com account, and see if you have entitlement to upgrade it to version 6 for free. See following KB for step by step process
VMware KB: How to upgrade license keys in My VMware
=== Update ======
an additional reference on this matter, might give you some more inputs.
VMware vSphere 6 Pricing and Licensing | United States
Narendra Padmani
VCIX6-DCV | VCIX7-CMA | VCI | TOGAF 9 Certified