I currently have a GlusterFS setup where the bricks are replicated, although brick 8 was having bad-sector issues, so should get replaced asap. After some searching I did not find how to do it... but is it pretty simple...
gluster volume replace-brick GlusterVolume node1:/usr/glusterfs/brick_1_8 node1:/usr/glusterfs/brick_1_13 start
Depending on the speed of the disks (2TB USB disks in my case) it takes some while, you can monitor the status via:
gluster volume replace-brick GlusterVolume node1:/usr/glusterfs/brick_1_8 node1:/usr/glusterfs/brick_1_13 status
Once the replacing is "done" you simply do a commit to finalize the replacing:
gluster volume replace-brick GlusterVolume node1:/usr/glusterfs/brick_1_8 node1:/usr/glusterfs/brick_1_13 commit
That's all 
