Update from 10.5.8 to Lion with the Lion USB stick I bought this small OS X Lion USB stick for $69 today. The guy in the Apple Store said that I can simply upgrade from my 10.5.8 to Lion with this thing. Well I plugged it in and hello, I'm not able to upgrade.

So question is: Can I even use this thing? I just have this stick, no serial, no nothing. Just this stick http://store.apple.com/us/product/MD256Z/A.
I know you can do a fresh install on a clean system, so you could try this: copy your existing install to an external drive (using Carbon Copy Cloner or SuperDuper!), wipe the internal drive, do a clean install of Lion, then use Migration Assistant to copy your old system onto the fresh Lion install.