External static IP change – Two connected servers

I recently managed IT in a small company and I am in the process of determining their configuration (they had previously outsourced IT work). They have a primary server (running Windows Server 2008 Standard) in one office and a second server (running the same Windows version) at the second office in another geographic location. All the files are stored on the main server and, after what I can say, the second one synchronizes with it. via a mapped network drive. Active Directory is also managed on the primary server.

We are considering switching to an Internet Service Provider at Head Office, which will result in an external static IP address change. Can any one give me advice on what should be done to maintain the current connection between the second server and the main one?