Comment 13 for bug 129392

Had the same problem.
Starting update manager as user or with sudo/gksudo didn't provide me with the upgrade button.

Adding

Dist: gutsy
Name: Gutsy Gibbon
Version: 7.10
Date: Thu, 18 Oct 2007 12:00:00 UTC
Supported: 0
Description: This is the 7.10 release
Release-File: http://archive.ubuntu.com/ubuntu/dists/gutsy/Release
ReleaseNotes: http://archive.ubuntu.com/ubuntu/dists/gutsy/main/dist-upgrader-all/current/ReleaseAnnouncement
UpgradeTool: http://archive.ubuntu.com/ubuntu/dists/gutsy/main/dist-upgrader-all/current/gutsy.tar.gz
UpgradeToolSignature: http://archive.ubuntu.com/ubuntu/dists/gutsy/main/dist-upgrader-all/current/gutsy.tar.gz.gpg

to the ~/.update-manager-core/meta-release as jared proposed did the trick.