When doing a hardware upgrade you are sitting on the inside of your network
When you remove the 2 MX 84. does the network still work for you so that you can add the MX250?
I am pre configuring my MX250
a.a.a.64/29
a.a.a.65(ISP)
a.a.a.66. (MX250 local config)
a.a.a.68 (MX250 future spare local config).
So
remove both MX84
add 1 MX250 ( I know to use similar ports then migrate to the SPF+ later)
Create the spare with a virtual config
add the second MX250
Remember to turn on VPN HUB.
This is just a rough start for migration but I am concerned how long I have before I have to. run and find a guest wifi ssid to add my MX250 since I deleted my MX84s.