Longhorn wasn't changed to vista, it was always a code name and never intended for release. Just like windows whistler and windows blackcomb were never intended to be release names.
For those curious, those names came about because there is a Longhorn Bar in the Whistler-Blackcomb ski resort in British Columbia.
Here is Wikipedia's list of Microsoft codenames: http://en.wikipedia.org/wiki/List_of_Microsoft_codenames. Strangely, the article lists Metro as the Windows 7 UI. Also, the XML Paper Specification (XPS) had previously also used the Metro codename.
