I don't think that these changes actually require re-learning the OS, though. They do require some learning, but "re-learning the OS" implies a much bigger learning curve than most people actually faced with the changes in the past. Yes, some people are greatly confused when, say, the control panel changes. But those people were already confused by the control panel. It's not as if these people were ultra-competent until the category view was added.

I actually agree that the control panel change is annoying, though. I think it was a bad change, because it's not merely different. I think it's actually less usable.

And yes, the change from Win7 to Win8 is massive. Not a lot of people would dispute that.