The above would only update a package if the centos repos had a higher version number than the installed SL one.... I would strongly suggest something akin to yum reinstall * and leave it to chug away (backups first naturally) for a while to refresh all the packages and teh rpm database to be in sync with the centos build..... requires matching, same build options for sure etc etc....
In the event something crops up it at least eliminates an odd untested mix for certain fundamental packages like glibc etc....
Yes, I know, for that machine, it's just a desktop that has x-window, gnome and a console opened up all day. I don't mind it if it function strangly... In any event, I'll just wipe the whole thing...