I made a long and detailed post elsewhere. The high points are:
The new model is moar better. More extended on top, better detail throughout the band, better background (or microdynamics), removable cable and nicer physical design.
They need a crapload of break in - a couple hundred hours at least - and as they break in, they may actually sound worse than straight out of the box.
I used a Richard Cheese album as an audio reference.
I was a hopeless fanboy for the 420, so if you don't like the 420, don't listen to me.
Shure SE-420 vs SE-425 (long) - Head-Fi.org Community