I upgraded a 8.0 (ftp) version to 8.1 (purchased "update" version).
It'll appear just like a regular installation.
For upgrading from ftp, use this
which only says to boot with "suse_update" option.
Basically, when installing, after 10 minutes or so, it'll ask you something like "upgrade existing linux" from either the ftp install or the purchased install. Just answer yes.
Also, for your reference, you can purchase the "upgrade" version from linuxcentral.com or from SuSE only I've found.
Also, upgrading is a popular topic in alt.os.linux.suse
You could search it in groups.google.com for more info.