Comment 18 for bug 1510841

Following the instructions in this thread https://ubuntuforums.org/showthread.php?t=2382832 to point /etc/apt/sources.list to the moved zesty sources allows the process to resume.

Surely the upgrade script could be modified to check if the current rev sources have been move to "old" and re-point the apt sources there? Otherwise if you don't know to upgrade within set time you get hosed into mysterious upgrade fail hell