Pro Sites: Where is the upgrade URL set?

I was eventually able to hide most of the references to Upgrade (a link still appears in the dashboard).

As I mentioned then, this particular website requires additional information (including a document that has to be uploaded and then manually verified) before an upgrade occurs. Therefore, I need to send the user to a custom form that was designed to accommodate that info/document.

While hiding links is certainly a helpful option, the "Upgrade" URL must be a Pro Sites variable, and I'd much prefer to simply change the Upgrade variable to a fixed URL that leads to the custom form. Then, the link in the dashboard would be useful.

Does anyone have knowledge of where it might be best to make such a change?