We have 2 DFS servers, both of which have MBR volumes actively replicating data between each other.  One of the MBR volumes is steadily approaching the 2TB limit.

Our plan is to add a secondary GPT drive, robocopy the data to the new drive, and then swap letters with the MBR drive.  We'll do one server at a time (obviously). 

My question is, when I flip the drive letters around so that the GPT volume is the active volume, is DFS going to see those files as new or modified and attempt to replicate?

Hi,

You will need to remove Server2 as a folder target before you are going to add the data on GPT disk. 

After adding Server2, an initial replication will still need to be performed. However if there is no much modification such as permission change, it should be finished quickly.