[CentOS] how best to rollback from a yum update?

Suppose I want to upgrade a bunch of packages on a system, but in case the upgrade produces unexpected, undesired results, I want to be able to rollback the system to its original state.  What is the best way to do that?

Often, I won't have, or be able to find, packages for the current installed versions.  I.e. If I haven't upgraded postgres for 2 years, it may be that I can no longer find a package for the version I am currently running.

The 'rollback' feature of rpm and yum [1] looks promising, but I have a few concerns with it:

a) why isn't the --rolback switch described in either the man page for rpm or the output from --help?  This makes me think the --rollback switch is deprecated or otherwise unofficial.

b) how can I trim/purge the 'repackaged' rpms that by default go in /var/spool/repackage [2]?  I don't want to just leave them there forever, but if I trim/purge/delete them, how will the rpm database be updated to know that it can no longer rollback to the date that they were installed?

AtDhVaAnNkCsE

[1] http://www.freedomit.co.nz/kb-centos/enabling-yum/rpm-rollback
[2] http://www.linuxjournal.com/article/7034?page=0,1
-- 
Jon Detert
Sr. Systems Administrator
Infinity Healthcare
Milwaukee, Wisconsin
414-290-6759