How do I change the audio on a H.264 MP4 without re-rendering the file? Is there any way to swap out the audio track on a rendered H.264 MPEG-4 without having to re-render the entire video project? Can this be done with Premiere, MPEG Streamclip, ffmpeg, or some other tool that I'm not familiar with ?

I have a long video project that took many hours to render, and I would like to swap out the audio (minor mixing changes) without having to re-render the entire video project. The new audio is exactly the same length.

Is this feasible? I'm primarily a Premiere user, but I'm happy to hear solutions for other tools.
ffmpeg is a good choice for this task. You can do this with this command:

ffmpeg -i input.mp4 -i input_audio.aac -map 0:0 -map 1 -vcodec copy -acodec copy output.mp4