I have an mp4 video file with an on-site audio track. I need to substitute a soundtrack score that I wrote into that same video file.

How can I swap the audio tracks without having to re-render (and therefore re-compress) the video? Is there a way to split the streams and then re-combine the video with a different audio track without rendering?

If I can do this in Vegas 8c Pro, great. I also have SUPER, if that tool will work for this task.