- Set them the same - 2013 can proxy to 2019
- If you are using a load balancer, then simply add the 2019 servers to the same load balancing pool that the 2013 servers are already in.
Since My goal is to have users access the same URL as they currently do, when I create the URLs in 2019, can they just be the same?
2013 = mail.domain.com
2019 = mail.domain.com
Or will that cause people to go to the new server before things are completely migrated?