This has been noted on the forum in a thread:
Claim:
That changing the Sources for Zorin OS to focal and then running "sudo apt update && sudo apt upgrade" will upgrade Zorin OS 15 to Zorin OS 16.
Status: False
Performing the above procedure will not upgrade Zorin OS 15 to Zorin OS 16. It will only break it.
If you think about it, this makes sense. If it was that easy, Canonical would not have needed to develop the Ubuntu Direct Upgrade build. And users would have been using the simple method for the last several years to easily upgrade releases.
Upgrading by release is far more involved than just changing the sources.
Please Do Not Fall For this Scam.