I did learn about systemctl switch-root after finishing my kexec solution, but I think if you're at the point of reimaging a server over the network, you probably don't care enough to shut it down correctly :)
Actually you do, bacause you can't re-image safely while kernel is also writing to your target block device via fs driver. And if original server has nything more complex going on, like md/lvm you're in even more trouble.