I'd recommend doing a clean Debian installation with the Sarge rc2 installer (pick the "netinst CD image"):
http://www.debian.org/devel/debian-installer/
Of course, you can simply edit the /etc/apt/sources.list file and make it contain only this one line:
Then do "sudo apt-get update" and "sudo apt-get dist-upgrade". There's a good chance that this will break your system but if it's a success, then you've just started tracking Debian Sarge instead of Ubuntu Warty.