Responding to duplicate post just in case...
Sounds like you've got the right idea, I would certainly connect the MX250 on its WAN1 interface only, perhaps from a lab or staging VLAN, anywhere it can grab a DHCP address (unless you've already followed the procedure and it's got static settings on the WAN side already) so it can contact Dashboard and pull its firmware and configuration (and possibly reboot itself during that process). This will certainly cut down on the actual cut-over time. Don't know exactly what your Internet segment network design is, but perhaps there's a way to run the MX250 in parallel for a short time, so you can at least have whatever LAN interface(s) connected and verify reachability and routing table before disconnecting the old equipment.