The result of having upgraded something only to find out afterwards that things are worse than before. Commonly used with the "-ed" ending and/or put in quotes to emphasize its difference from a real upgrade.
Bill: Bob just upgraded his WinXP box to Vista... well, "updegraded" it to Vista.
Jim: Adobe Acrobat Reader used to take about 5 MB of disk space and was very fast to load; now it's slower and uses 150+ MB!... yet another "updegrade".
Sam: We used to have the Bill of Rights; now we have the Patriot Act. UPDEGRADE!