[Centos] Using yum to do downgrades

This is a yum question but relates to some work I am doing with CentOS 
and other recent discussions on this list so I though I would post here 
first.

Basically I am attempting a Red Hat 7.2 -> CentOS-2 upgrade.  So far it 
has worked like a charm.  The problem is though that yum (no doubt by 
design) will not downgrade packages to the CentOS versions.

The problem is that some old RPMs have 72 in the release (for Red Hat 
7.2) and the newer package as 21 in the release (For Enterprise Linux 
2.1).  Some others are updates to 7.2 which increased the version number 
where as 2.1 got a backported fix and increased release number.

Is there a way to make yum always select a particular Vendor or 
Distribution?  What about using yum to do an explicit downgrade
"yum install mypackage-version-oldrelease"?

Also, this is an rpm question, does anyone know if I can do
"rpm --erase redhat-logos" and "rpm -i centos-logos" at the same time?

If I put an obsoletes: tag into centos-logos will yum do that for me? 
Is that a good idea?

At the moment I have a bash script which trys to work out the downgrade 
rpm commands but I can not check dependencies.  Any other suggestions, 
tips or tricks would be appreciated.

John.

-- 
John Newbigin - Computer Systems Officer
School of Information Technology
Swinburne University of Technology
Melbourne, Australia
http://www.it.swin.edu.au/staff/jnewbigin