There is no difference on license between 5.0, 5.1 or 5.5, your license for version 5.0 can be used for any version of vSphere 5.x.
About the upgrade, you will need a Support and Subscription contract active to upgrade your license for a major release of vSphere, like the version 6.
Here are some links about license upgrades and licensing of vSphere:
Note from above link:
- The vSphere 5.5 does not need new license keys. The existing 5.0/ 5.1 licenses will work and enable all the features of the software once upgraded
Senior Infrastructure Specialist
Thank you for your links and response. So what I understand now is that even if our contract is expired still we can download 5.1 or 5.5 and use same product key of 5.0 to unlock new software ?
Please correct me if I understood wrong.