Extreme change is mostly bad since people don't like drastic changes. I mean, just imagine that you come home and your girlfriend changed the entire living room. Before she changed the entire room you felt safe, knew where to find stuff like the remote control and could complete the "task" of turning on the TV.
Now, even though she bought nicer looking furniture, a more comfortable couch and a bigger TV you have a hard time finding your stuff and don't feel "at home" because it's all new and overwhelming.
I think you get the point of the example.
I still love this example from eBay:
At eBay, they learned the hard way that their users don’t like dramatic change. One day, the folks at eBay decided they no longer liked the bright yellow background on many of their pages, so they just changed it to a white background. Instantly, they started receiving emails from customers, bemoaning the change. So many people complained, that they felt forced to change it back.
Not content with the initial defeat, the team tried a different strategy. Over the period of several months, they modified the background color one shade of yellow at a time, until, finally, all the yellow was gone, leaving only white. Predictably, hardly a single user noticed this time.
There are many more examples like this one.
Even though this article is from 2006 it's still a great read: