The newer Shure headphones have a connector where the cords from each earphone meet, if that makes sense. From there, they attach to a cable which can be swapped out which plugs into your device. With the SE series, you just swap out the bottom cord for an MPA.
I had E2Cs (I think) and switched to the SE210s just to deal with the long cord situation, but had someone who wanted my old earphones.
For you, I'm sure Shure would love to take $400 off your hands on some new SE420s...