Okay I have learnt the hard way!, I did a release upgrade over Byobu, It all goes well until later on in the upgrade when the byoubu package itself gets upgraded and then its game over you lose the session.
However
I had to do a few of these upgrades so I ran up a 10.04 LTS VM and this time put byobu on hold via dpkg
# Put byobu on hold with dpkg
sudo su -
echo "byobu hold" | dpkg --set-selections
#check it is on hold
dpkg -l | grep byobu
byobu hold
exit
and ran the release upgrade again. The release upgrade honored the hold and left Byobu alone. afterwards when i was in my new 12.04 system I could release the hold
echo "byobu install" | dpkg --set-selections
and then just install the latest version of byobu
sudo apt-get update && sudo apt-get upgrade