When should a UI be updated?

I'm working on an online tool for students in my faculty to calculate their final grade for their degrees, and for a while I've had little problems. Students seem to like using the new UI and I've suffered no criticism for a while. Now, I need to add some functionality to the program, meaning the UI will have to chance slightly, but in some very noticeable places.

After watching the whole New Facebook thing kick off, along with the slating of applications with bad UI here on SO and the Ribbon on MS Office receiving mixed reviews I've wondered to myself whether a UI should ever really be changed if users seem to like it. Should a UI be updated after a number of years to keep users interested/be more efficient? Should features be held back in sake of a clean UI?

From stackoverflow

• Ideally when you first design your UI you create something flexible enough to add any feature you want in the future. In the real world, this is seldom the case. I would say UI updates should be slow and incremental if at all possible (that was the real problem with the facebook change, one big change all of a sudden), and if not then people should be given plenty of notice of the change, and while you're changing you should be sure to plan for future additions so as to not annoy your user base more often than absolutely necessary.

• That's a tough one.

  Sometimes changing the UI to accommodate new features can't be avoided, but if you are really concerned about the impact of those changes, you might want to step back and try to re-state your problem. Is there another way to handle the features? Is there a way to lessen the impact on the UI? You might even want to try to re-think the whole UI and do some mock-ups that incorporate your new features as if you are building it for the first time. You don't have to make the change, but the exercise might help you think about the problem in a new way. You may also find that the new UI works better, and if you make it intuitive enough, the change might be welcome. If you were to go down that path, I would try a pilot, get some feedback and be prepared to go back to the old UI if it goes badly.

• Funny. I'm in that very spot right now. Our web-based system has a UI that was designed around solid usability concepts and the users LOVE it. (over 1000 users and lots and lots of feedback).

  But the marketers hammer me about once a year to make the system "more sexy" because "they can't sell it". Now I maintain that given the user feedback, they are perhaps marketing to the wrong audience if only "sizzle" will win them over, but still...

  I let them loose withing some carefully chosen parameters to redesign the main UI (menu look and feel, logo placement), and if it does not negatively affect the usability, then I make the changes. As I said, it happens about once per year.

  What is interesting is two things.

  1. Because no real usability items get changed, the users still love the system.
  2. Just making subtle changes to the general "look and feel" makes the sales folk deliriously happy.
  3. The users actually respond very positively to the changes as well. They feel as if we're "keeping current" each time we change, but always mention that they LOVE the fact that we don't tamper with the working of the site.

  Cheers,

  -R