You are correct: For Pro users, the download link for 17 will remain valid for 17 point releases only.
Upgrading to Zorin OS 18 can be done through the Upgrader tool, but only to 18 Pro with the purchase of 18 Pro.
IF a Zorin OS 17 pro user wants to switch to Zorin OS 18 Core, they will need to perform a fresh install.
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