I would advise to keep the old ISP plugged in and configure new ISP as well. MX64 has dual WAN uplinks.
Once the new ISP is configured an in ready state, remove the cable for ISP 1.
Also check if you have any configurations in SD-WAN & traffic shaping, to be moved to WAN2 since the new ISP will be on WAN 2.
If you are unable to do it as mentioned above and would simply like to replace the ISP cables, remember you have access to local status page of the MX to configure static IPs incase the device goes in a state where you are unable to access it via dashboard.
You can access the local status page by navigating to http://mx.meraki.com/ by connecting to the LAN interface.
Follow this documentation for local status page. Both methods are good, and its just about preference.
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