How to track two versions of website running concurently
We are redesigning our website and are looking to launch it shortly. When the new website is launched it will run concurrently with the old one for at least a few weeks. The domain/hostname for both sites are the same. After a few weeks the old site will be retired.
In GA there is a property and views for the old site that has been running for a long time, and a GTM container. When work started on the new website a new GA property, views and GTM container was created for it.
Ideally I'd like to continue using the old (existing) GA property, views and GTM container so that when I run reports I can still use historical data. For example, before the transition to the new website there were x sessions per month, during the time both old and new versions of the site were running concurrently there were y sessions per month, and after the old website was decommissioned there were z sessions per month.
I can simply make a change on the new site so that it runs the same GTM snippet as the old site, so traffic data from both sites will flow into one property-the old one. But how can I distinguish between traffic from the old versus the new site? Hostname is the same for both so I can't segment by it.