Did you try rebooting the MX?
No Adam, I thought so reboot might be needed, but doesn't looks good that rebooting MX just for clearing arp. Actually had an issue with IP conflict, source switch rebooted and issue resolved.
That is inexcusable. I am replacing another vendor with a Meraki switch that connects to an MX. We will have to reschedule to off hours just so I can reboot the MX to clear the ARP table.
To reply to my own post, we just used a different IP address for the new device. We changed the next hop on the MX. We did not reboot the MX in a maintenance window in order to clear the ARP cache.