I think you will have to do a cold swap. It involves removing the existing MXs from the network and then adding the new. You will need to re-set any static IPs if you have set them on interfaces and re-enable the site to site VPN, but pretty much everything else moves over. The only other thing that normally needs doing is mapping old to new ports, but you are not in routed mode with VLANs, so that shouldn't be needed AFAIK.
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