You should plan for it all to break as soon as you remove the MX84's from the network. Use a hotspot off your mobile phone to maintain connectivity to Dashboard while adding / removing.
The second option would be to clone the existing network and put the MX250's in the clone. When you clone it will leave AutoVPN off. Once the MX250 are up and local internet is working you can disable / delete the old MX84 network and bring up VPN if needed.
Yes you still need to setup the WAN links on both MX250 locally before they will connect out to dashboard to grab their complate configuration.
The cloned network operates exactly like any other network, so same warm spare options etc. The reason for cloning is to keep the original network unmodified so you also have a rollback plan.
To keep the public IP addressing unique you would need to power off the MX84 if you don't have spare IP addressing in the public IP allocation from the ISP. WAN configuration IP addressing isn't validated and checked by Dashboard like LAN side settings are so no problem with duplicates.