You can't. Several people have shared stories about trying to get it in writing from Comcast (and it is always Comcast) that service is available at a particula

You can't. Several people have shared stories about trying to get it in writing from Comcast (and it is always Comcast) that service is available at a particular address before they move there, specifically to avoid this issue, and the CSRs will not do it. They're very careful not to make a paper trail with discoverable evidence.