I think jackd/libjack and their dependancies/installation have changed so much since U6, and that you have two choices,
1. Back up your U6 as needed, and do a fresh U7 install
2. Use synaptic or similar to remove all jack related things
from your U6 system before 'upgrading'. And first, confirm that your
priorities are fully supported by the U7 you have, and that you are
not defeated by hardware or software not fully supported out
of the gate.
I find that upgrades that traverse multiple releases are bound
to break something you like, and since so much has changed in the
several Ubuntu releases, you may as well install a full new version,
and then add your preferences when you have a working install. When
upgrading skips several KDE or Gnome releases, even the eyecandy and
settings in your /home/user may fail to compute, another vote for a clean install. Mepis was the distro that seemed most proficient at preserving a working /home partition, usually only a couple KDE tweaks needed re-enabling, theme, mail and browser etc were all ready to go
after a new install. Hope this helps.