Which TV show changed the most from its first episode to its last?

A new TV show is like a shiny toy, mint in the box: You can't wait to start breaking it and rebuilding it. Any show that stays on the air a few years will have cast changes, but also mission creep and radical status-quo alterations. Which science fiction or fantasy show changed the most from its first episode to its last?

Please include a picture or clip for your chosen TV show, and please mention the name of the show and what changed from start to end. (Thanks to Straight Dope for the idea!)

Top image: Angel, the beginning and the end.