Comment 5 for bug 129392

It looks like the workaround found by Jared offers you a one-time only opportunity to upgrade.

For me, it worked the first time I tried, but I didn't have the time to do the upgrade at once. But when I came back to do the upgrade, the button was gone, no matter that my ~/.update-manager-core/meta-release file is up-to-date with the text from http://changelogs.ubuntu.com/meta-release-development

I also tried to update my ~/.update-manager-core/meta-release file again from the given url, although the text appeared unmodified at eye inspection, but of course it just didn't work because the "new" text was indeed unmodified...