Replacing old freepbx with new appliance Freepbx

I am trying to replace an existing pbx with a new freepbx appliance.
Ideally I would like to use the same IP addresses on the appliance so that I can make a seamless transition.
I have eth1 configured with an external ip and eth0.20 configures with a vlan.

When I plug the eth1 to the same port on the isp modem device I do not get a connection. I believe this is due to the modem device caching Mac adresses.

To get around the problem I tried Mac spoofing so i put the same mac on the new device and i disconnected the old device. When I did this the appliance did not seem to work very well. The sys admin module said that I was not activated and the whole system responded very slowly.

What is the best way to go about doing the switchover? Does the appliance block Mac addressing spoofing?