installing an old version of an update

Hi.

I'm running centos 4.X with yum 2.4.3.  I'm trying to install an older 
version of an "update" from the yum update repository.  The problem is 
that when I do "yum list all", I don't even see the update even though 
the file is there.  In my particular case, I have a script that I use to 
   install a specific set of updates on machines.  It works as long as 
new versions of those updates are not available.  I've tried specifying 
the full version/arch/etc, and it still doesn't work.  Just because a 
new version of an update comes out and is in the repository, I don't 
want it being installed on all of my machines until I've had my own time 
to test it.

Any assistance would be appreciated.

Jason.