Hmm.... i would say the answer is "it depends" :smileyhappy:
From a technical standpoint the "yes" is right and the vSphere 6 license key work for every 6.x version.
Its a "no or depends" from a legal standpoint. If you bought vSphere 6 three years ago with 1Y subscription you are only allowed to run 6.5, 6.0 and older but not 6.7 because your subscription timed out before the release of 6.7.
For me your question was not totaly clear and i often have to deal with customer who runs out of subscription.
Regards,
Joerg