IP address change

I have setup freepbx and made several tests and everything seems to work. I am coming in this weekend to go live with it and replace our current Fonality system. I have been asked to change the ip address of the freepbx server to what the Fonality system has now when i go live on Saturday. Has anyone changed the server ip and if so did you run into issues with freepbx? The reason they want the ip address to be what the Fonality server has is due to us being connected to other sites so we can make VOIP calls.