My lessons learned from replacing MX's so far.   1.  You can add them as a secondary warm spare device in the current network.  It won't take an additional license and you can plug it into any internet connection so it can get its initial config. 2.  If you are going to be replacing an existing device make sure to setup any static IP information on the new MX.  That information won't transfer over.   3.  When ready you should be able to move all connections from the old MX to the new MX in a 1:1 fashion.  4.  After the new MX is online you can remove the old MX.  The only other things you may have to configure on thew new MX is the device name, address/location, tags etc... ... View more