Thanks for accepting my answer as solution, but Kudos should go out to @jdsilva who also provided a thorough explanation! 👍
Non-local configuration (like firewall rules, routes, Vlan settings) will be retained by this method:
"A Dashboard Network can only contain one MX at a time. To make room for the new MX, the Network Administrator will have to remove the current MX. Please note that the network will retain the old MX's non-local configuration, so the replacement MX does not need to be reconfigured in Dashboard."
Please also keep that one in mind:
"Because the new MX will be running an exact copy of the original MX's configuration all cables should be moved to the same ports on the new MX as they were connected to on the original MX. For example, the cable from LAN port 4 of the original MX should only move to LAN port 4 of the new MX, or another port with an identical configuration.
Once this has been completed and the new MX has fully checked in and finished its upgrades it should now be in place and functioning on the network with the exact same configuration as the original MX."
So basically, all you'll have to do is set up your WAN interface (in case of DHCP even that should be transparent), let the new box pull the config from Dashboard and be all set. 🙂