If you don't have a subscription anymore and that's not likely to change, and you don't care about losing Redhat's (or other vendor's for that matter) support for this box in the future, you can switch it to CentOS and have your box updated with its packages. It's not ideal, but it may still preferable to having a box you can't update anymore.
You have been warned.
"All that you should need to do is install centos-release, remove
redhat-release rpms and just yum update the machine, which should
bring in all packages changed by CentOS ( since they will have a
slightly higher E-V-R )." - per KaranbirSingh on centos-list
You may also wish to remove the "rhnplugin" from yum (not necessary but gives an error msg)
OR - per Craig White on centos-list Mon, 29 Oct 2007 (updated for
# cp /etc/redhat-release /etc/redhat-release-saved
# rpm -e --nodeps redhat-release-notes redhat-release yum-rhn-plugin redhat-logos
# rpm -ivh \ http://mirror.centos.org/centos/5/os/i386/CentOS/centos-release-5-8.el5.centos.i386.rpm \
# yum update
Link to the fixed package announcement in CentOS: