While it is rather easy to change the hostname of the Linux box the GlusterFS is sitting, Gluster might not be so kind. Good thing there are people who already had to scratch their heads.
# First stop the service
systemctl stop glusterd
# Replace all old names with the new one
cd /var/lib/glusterd/
find ./ -type f -exec sed -i 's/$OLD/$NEW/g' {} \;
# Also rename any folders and files with the old name.
# Be careful to get the right filename, they differ based on the mount point
cd /var/lib/glusterd/vols/<volname>/
mv ftp.<old_name>.mnt-gluster-san.vol ftp.<new_name>.mnt-gluster-san.vol
cd /var/lib/glusterd/vols/<volname>/bricks/
mv <old_name>\:-mnt-gluster-san <new_name>\:-mnt-gluster-san
# Restart GlusterFS
systemctl start glusterd