[mbilker]

ffmpeg can copy streams without transcoding. For example: ffmpeg -i input.m4a -c:a copy input.mp4

[deadf00d]

In this case both mp4 and m4a (which are the same thing) use AAC. So no transcoding, but mp3 is not. So in the case of mp4 > mp3 it need a complete re-encoding of the audio.