Lustre 2.1.6 server and 2.5.3 client.

Do they compatible ?
The answer is yes!
Lustre client upgrading using yum in centos 6.5:

vim /etc/yum.repo/lustre.repo
[hpddLustreserver]
name=CentOS-$releasever - Lustre
baseurl=https://downloads.hpdd.intel.com/public/lustre/latest-maintenance-release/el6/server/
gpgcheck=0
enabled=1
 
[e2fsprogs]
name=CentOS-$releasever - Ldiskfs
baseurl=https://downloads.hpdd.intel.com/public/e2fsprogs/latest/el6/RPMS/
gpgcheck=0
enabled=1

[hpddLustreclient]
name=CentOS-$releasever - Lustre
baseurl=https://downloads.hpdd.intel.com/public/lustre/latest-maintenance-release/el6/client/
gpgcheck=0
enabled=1

On client:
1) yum update lustre-client -y
2) and finally one liner to restart the lustre mount point:

umount /lustre;lustre_rmmod;service lnet stop;service lnet start;mount /lustre