What does Virtualmin do to make "Website Disabled?"

I’m trying to move all my websites to an upgraded server but need to do them a few at a time but without incurring much downtime. As both servers are behind a firewall which routes port 80/443 to the web server, I plan to backup, disable a website, restore it to the new server, then add a proxy rule for apache on the old webserver to point to the new webserver for that site. However, nothing I add to the apache config seems to work and the website simply displays the “Website Disabled” message. Can anybody offer some advice please?

I found the answer, it was in each Virtual Server’s apache config file (AliasMatch). However, this is what I really needed: https://www.virtualmin.com/documentation/system/migrate