Is there any real reason why you need to move the uplink to WAN1 instead of WAN2? If not, you could just leave it as configured and save yourself an outage.
To change the WAN port settings:
Check your dashboard now. If you see the following message on the Uplink tab for the MX, you'll need to configure it using the local status page:
NOTE: Local WAN configuration cannot be overridden on this firmware version.
So long as the MX is accessible from within the network and you have the local status page enabled under Network-Wide -> Configure -> General, you won't have to have physical access to the MX to make the interface change.