According to I bought VMware Workstation 16 at last month, I want to know can I downgrade license key from Workstation 16 to Workstation 14? because I can't find in KB.
I found about downgrade only version Workstation 15 and 14 form this link
Workstation 14: https://kb.vmware.com/s/article/2126721
Workstation 15: https://kb.vmware.com/s/article/56987
That should be possible: https://kb.vmware.com/s/article/2006975
Why do you want to run Workstation 14?
VMware Workstation 16 doesn't support Windows Server 2008 R2 so I have to use Workstation 14. From your link, I found only how to downgrade... but it don't show about workstation 16 downgrades to workstation 14.
You asked about downgrading your licence key, the link I posted is how to do that for all VMware products through My VMware.
There is no downgrade of the actual software, you would need to remove v16 and install v14 (once you have the licence key)