yes, rsync against the old ISO's will use much lesser bandwidth.
If I can rsync and get the changes with just a little bandwidth, that will leave that much more left over for me to run bittorrent and get the ISOs to people who don't have them.
'yum upgrade' does a good job of moving you from Beta to RC1. However, keep in mind that there are a significant number of package changes.
But my goal is to help distribute the CDs not just update one machine.