The reason people keep talking about a USB connector is that the AUX input simply amplifies out of the speakers the audio that the MP3 player produces. In other words, it uses the MP3's amplifier, not the car's, resulting in extremely poor audio. In contrast, a USB connector simply takes the data off the device and uses the car's amplifier so the audio is as good as if it was coming of the radio or CD. You also have the benefit of navigation using the car's controls, whereas the AUX input does not provide this ability. It is passive.
I really think you could replace the whole switch panel containing the new head unit, but it would probably be expensive. Any one have a good source of discount OEM parts?