Are new licenses required for this or is this just an upgrade to a newer version?
As mentioned before, each major version requires it's own license keys. Anyway, if you have an active subscription, or even had an active subscription when vSphere 6 went GA, you may be eligible for a free upgrade. The easiest way to check this is to login to your MyVMware portal, and select "Upgrade" from the license page, and see which licenses keys show up as upgradable.
André