I think it does not make much sense to reinstall all packages. Both CentOS and Scientific Linux aim to be binary compatible with RHEL. Anyway, all update packages come from the Scientific Linux repository after the “conversion”.

You can see the list of changed packages compared to upstream:

Scientific Linux 6.0 Release Notes
Scientific Linux 6.1 Release Notes
CentOS 6.0 Release Notes

You may want to reinstall only the modified packages. This may be (partially) achieved using yum distro-sync.

Quick and dirty for loop to reinstall all of the CentOS packages if people are that way inclined

for i in `rpm -qa –qf “%{NAME} %{VENDOR}\n” |\
grep -i centos | awk ‘{ print $1 }’`
do yum -y reinstall $i
done

The list of packages modified by CentOS is here. To replace the CentOS package with the SL one please run yum reinstall package.

Run rpm -qa --qf "%{NAME} %{VENDOR}\n"|grep CentOS to see the packages created by CentOS. It is not necessary to reinstall all of these packages, CentOS and Scientific Linux’s aim is to be binary compatible with RHEL.