If you download ubuntu 9.10 RC iso file, You can use zsync tool update your RC iso file to Final iso.
sudo apt-get install zsync mv ubuntu-9.10-rc-desktop.iso ubuntu-9.10-desktop.iso zsync http://ftp.tw.debian.org/ubuntu-releases/9.10/ubuntu-9.10-desktop-i386.iso.zsync
Link:
Use zsync to update existing .iso images (alpha, beta or RC -> final)