Use the net install CD, I further suggest downloading the full DVD iso on your hard disk, in some filesystem, in a simple location (for instance, use /dev/sda1 mounted on /, and the iso in the path /oo11.1.iso).
At some point in the install (maybe at the boot screen, but I'm not sure), you'll be prompted on whether your installation sources are located on the disk or on the network or somewhere else (choose hard disk). I then remember you'll come up with a text GUI screen, where you can quite easily instruct the setup program to pick the sources from the disk. Tell it to use the disk where the ISO is located (/dev/sda1 for instance) and the path (/oo11.1.iso in my example). Then, you can choose to begin the real installation, and you'll be brought to the usual install screens, choose "upgrade" as usual and then everything will go as fast as if you were using a DVD.
I use a very similar procedure when I want to install a fresh new SuSE, although I do not use it to perform an upgrade, and it works without problems.