To replace a class with a similar one right now you have to find and click the element, remove it's class, add the new one. Why not offer a simple "replace class" action in the navigator next to a class, where an input field appears on click. The same "create class" UI can be shown in-place that is available in the first pane.
Imagine an elaborate navigation menu, where the active state is very different from the default state. Right now I have to go to the menu, click around until I hit the correct element, remove the class, add the new one. If I could just go into the navigator, click to replace the class and do so just by typing the new class name this tedious task can be done quicker.
When you finally offer free class combination this idea can also be used to add new classes via the navigator.