We purchaed Vsphere 5.5 Enterprise Plus license and willing to upgrade to 6.0, Need to clarify if license of 5.5 can be used in 6.0 OR need to buy fresh license, since it is version change.
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
=== Update ======
an additional reference on this matter, might give you some more inputs.