Enhancements aren't added to existing ArcObjects, but to new versions of the same object with a number suffix. For example, the ILayer interface was created in the beginning. Later on, there were enhancements added to that interface, such as changing the AreaOfInterest property from read-only to read-write, as the ILayer2 interface.
One thing to note is that you can't always use the enhanced object, since it may not be implemented by all the classes of the original. Many more classes implement ILayer than ILayer2. For example, you can use ILayer on a RasterLayer, but you cannot use ILayer2 on it.
