Have you ever radically changed campaign premise mid-stream?

For instance, the heroes of your fantasy epic suddenly 'wake up' and discover that they are characters in a Cyberpunk campaign (I stole this shamelessly from Red Dwarf - although that wasn't exactly a fantasy epic)?



I honestly don't know how I would react to that kind of switcheroo. It would take one hell of a GM to pull that off without alienating the players. What do you think?

Have you ever radically changed campaign premise mid-stream?