Ok, I figured it out. It looks like the Fences upgrade does not consider the current email address attached to your Stardock account, even if you are logged in and initiate the upgrade from there. Instead, it is looking for whatever email address you may have had when you first registered Fences. For me, that was 5 years ago, so I'm lucky I remembered.
This seems like a bug in the web form or an issue with how Stardock is managing account data. At a minimum, you should warn users that you are looking for the OLD email address, not the current email address linked to their Stardock account. Ideally, you should fix this to enable the check for both the original email address user to register OR the current email address associated with the account of a valid license holder.