Existing network (firmware latest as of 10/09/2016, controller ver. 5.29)
1. Make sure your NETWORK under Settings is setup how you wanted. For example, internal ip to 192.168.5.1/24
2. Forget old USG3 (have to do this or you cannot adopt a new USG; only one USG per controller)
3. Change computer IP to that of the default subnet in 192.168.1.1
4. Add new IP address via SSH.
b. Set interfaces ethernet eth0 address 192.168.5.1/24
i. NOTE: You set it to your desired IP address which you configured in step 1. Eth0 interface
is the first LAN1 port in the USG Pro 4
5. Adopt USGPro4 in Controller GUI
6. Change computer IP to the NEW IP subnet range, example 192.168.5.100
7. Double check the new USGPro4 has been adopted
8. I might have used the Controller GUI inform command, someone chime in if needed for other users
9. SSH into USGPro4
b. Delete interfaces Ethernet eth0 address 192.168.1.1/24
c. Commit -> Save – Exit -> Reboot
10. Wait for it to do it’s thang and boom, adopted and connected.