I also managed to resample my entire portable playlist (yeah, making mp3s out of mp3s, I know in some circles I could be killed just for thinking about that:*) to overcome that dreadful loudness craze that makes pretty much every track produced in the last 20-25 years destroy your ears if you happen to listen to it just after something 10 years older. Unfortunately all methods to level the tracks (mp3gain, replaygain etc.) don't work properly because they should analyze all tracks dynamic content, not just the average/maximum level, then first adjust their dynamics and later normalize their level. In other words, level compression should be applied more to softer tracks and not the other way around. So I faked a solution to the problem by reconverting my playlist after applying compression to an extremely low threshold, making clear to myself that none of those tracks would ever leave my player. Some sound really horrible, but I spared my ears.
This Wikipedia article about loudness wars explains the problem in details. https://en.wikipedia.org/wiki/Loudness_war
You can't 'uncompress' an MP3 back to having the dynamic range it had before.
True. In fact I would rather compress old music to bring it to the same dynamics of the new one, then and only then normalize everything. I know it's almost a crime; I did it just on my player and would never give these tracks to anyone.
Because I don't think we can.
Certainly, you probably need lossless if you're going to do any sort of editing.