For the record, one more successful upgrade from 6.06 to 8.04 to 10.04.
I only had some issues with GRUB.
At the first step, it was the switch to UUID notation. The upgrade commended out all old entries in the fstab and replaced them with UUIDs, but didn't do it for the grub menu.lst (or maybe I choose to keep the old grub configuration). I had to replace the old entries with the UUID in menu.lst and then I could boot normally.
At the second step, GRUB's menu didn't show the new kernel at all. I added it manually and it worked. Afterwards, I upgraded GRUB to GRUB2 and we'll see how the upgrade to 12.04 goes.
Thanks for you help!