If your MX84 is using AutoVPN (Meraki to Meraki site-to-site VPN), you might need to turn it back on again, but the underlying config will still be there; under Security & SD-WAN > Configure > Site-to-site VPN make a note, on the MX84 setup, as to whether it's set Hub or Spoke, then re-apply this after you put the replacement MX in.
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