This notes comes from our experience upgrading our Tier3 from 1.2.x. Based on the OSG instructions (references) and adapted to our topology (rocks 5.4). For updated instructions check first OSG instructions.
https://www.opensciencegrid.org/bin/view/Documentation/Release3/HadoopOverview
https://www.opensciencegrid.org/bin/view/Documentation/Release3/UpgradeHadoop
Make sure to remove repository for hadoop-0.20 Caltech.
yum remove osg-hadoop
For datanodes a local mirror is faster.
wget http://download.fedoraproject.org/pub/epel/5/i386/epel-release-5-4.noarch.rpm
rpm -i epel-release-5-4.noarch.rpm
yum install yum-priorities
rpm -Uvh http://repo.grid.iu.edu/osg-release-latest.rpm
#or use /etc/yum.repos.d/osg.repo
#with osg.repo pointing to a local mirror
umount /mnt/hadoop
/etc/init.d/hadoop stop
install hadoop
yum update hadoop-0.20-osg
#recrate broken link
ln -sf /etc/alternatives/hadoop-log /var/log/hadoop
#local configuration /etc/hadoop-uprm
#rm -f /etc/alternatives/hadoop-etc
#ln -s /etc/hadoop-uprm /etc/alternatives/hadoop-etc
#start hadoop
service hadoop start
#mount fuse
mount /mnt/hadoop