I'm unsure your exact question but here goes. The MX64 allows for 2 WANs, you change port 4 from a LAN to WAN on the main Appliance Status page. You can then activate WAN2 and make sure it works, Once working you can remove WAN 1.
Technically for the brief moments both WANs work it could failover but WAN 2 can be the primary WAN with no WAN 1 setup.
If you are onsite you can access the local status page and locally just update WAN1 to the the new info and then change the cables.
There are probably a couple of other order of operations you could do. I do recommend if you only have 1 WAN to eventually have it located in WAN 1 on this model as the dual nature of the second WAN can sometimes be problematic if you ever need to reset things remotely. The Port can revert to lan functionality. It's rare but it happens.
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