Comment 3 for bug 1537943

"The only possible upgrade path from 14.04 is via 14.10" - this is not the case as the release upgrade process has been modified to skip over releases that have reached an End of Life, like 14.10 has. That being said there is currently a failure to calculate the upgrade from 14.04 to 15.04 as documented in bug 1534374.

The release upgrade process will not be modified to support upgrading to End of Life releases as it is likely they have not received security updates for critical issues.