If anyone is having trouble doing the root to root rsync because maybe they disabled root login on the old server, or you just want to do this as a non-root user, I found this post to be helpful to figure out how to use sudo on the remote server: permissions - Using rsync with sudo on the destination machine - Ask Ubuntu
Let’s say you have a user, discourse, on both sides that have sudo privileges. On the remote machine, you’re going to edit the /etc/sudoers file with sudo visudo. You’re going to add the line:
discourse ALL=NOPASSWD:/usr/bin/rsync
Then on the new machine, you’re going to run (as your non root user):
sudo rsync -avz --delete --rsync-path="sudo rsync" discourse@old.ip.address.here:/var/discourse /var
This will allow you to run everything described here as non root users. If you’re keeping the old server around, I’d go back into the /etc/sudoers file and delete the line you just put in.