And only change what are real problems. Maybe something like a user needs to navigate to three different pages to get the information they need. That's something you can improve.

But resist the urge to do a general facelift or reorganization just for cosmetics. Users can fly through the ugliest interfaces, even text-mode terminal interfaces, once they get used to them. Change that at all, and they will be stumbling around and complaining. That can be worth in the end if their usual workflows are now shorter and faster, but tread carefully. Users of enterprise apps really don't care too much about how they look.