When is it profitable to redesign your interface?

Rumors are spreading that Microsoft is planning to completely overhaul their new interface for the Office series of programs. Great. That's the last thing I need is to have a "sharp learning curve" for the technologies I NEED to function every hour of every day.

Woah. Did I just say that? Can there be such a thing as techno-addiction because apparently I've got it.

As I was thinking about this new interface design and the pain it will cause me and millions of users, I began wondering why Microsoft would do this. Despite competition, MS has a pretty solid consumer base, so they don't need to redesign the interface to stay profitable necessarily. So why tick off all your users by redesigning what everyone is comfortable with?

I don't get it.

And this led me to another thought: At what point is redesigning your interface warranted by the return on investment you'll receive? For example, MS must be expecting some benefit from redesigning their interface--will that benefit be enough to compensate for all the ticked off users who had to learn the new interface? At BYU, the library has become very well known for redoing their website's design nearly every year. Each iteration of the design is supposedly more usable and "better", but everyone on campus has such a poor opinion of the library's website, and in interviews I had a couple of professors explain that their productivity was impacted because of the annual learning curve of learning the website all over again.

So, at what point can you expect the benefit to outweigh the difficulty you'll create for your users? It seems that many times, even if the new interface WOULD be better (which is often a debatable assumption), it may be better for your users if you just left good enough alone and didn't mess with the design anymore.

That's my takeaway from all this--to be careful in my designs to limit the desire to innovate, change, and redesign so that, above all, it's usable and familiar to my users because it's intuitive and similar to what they've used before.