[CentOS] upgrading 3.6->4

greetings - I'm trying to upgrade a centos 3.6 box to version 4.  I 
tried to do an upgrade using yum but that failed and now the Cd 
installer is not letting me upgrade either.  Here's what I did:

following the howto, I tried to do the upgrade using yum.  I installed 
the GPG key and upgraded the centos-release and the centos-yumconf 
packages.  When I tried to upgrade yum package I ran into dependency 
problems.

I downloaded and reinstalled the version 3.6 centos-release rpm.  Then I 
downloaded and burned the CDs.

I boot from the CD but I don't see an option to upgrade.  I get to the 
installer screen that asks for the type of installation (desktop, 
server, workstation etc) but I see no option to perform an upgrade.  Is 
the upgrade option hidden somewhere?  did the installer not detect my 
centos 3.6 system?  When I tried the yum upgrade, did I do something to 
make the installer think I have centos 4 installed (like the GPG key)?

Any help is appreciated!
cheers
Mike