You are correct Nothing will break (unless your IP Addresses are changing if your using statics?), and your MX80 will still work, and it will max itself out (software or hardware, not sure honestly) and you basically just won't get more than 250Mbps. When you install the MX84, you'll see the bump in performance on the WAN circuit. Make sure that slider bar is set to MAX, sometimes it keeps the old MX config (250 in this case). I don't think you'll have any issues with dropping anything personally. ... View more