As a huge Hifiman fan, I didn't particularly care for the RE400's. Mainly in the treble region. They are still a fantastic earphone but if they tuned the midrange and the treble same as the RE272 and kept the RE400 bass, that would have been great.
The M200 will give more bass, especially in the sub-bass region. I'm not about to go into details on the rest since I haven't heard the 400's in awhile but the M200's are going to be less flat sounding and more colored compared. I think it's more engaging and agrees with most genres of music without being artificial sounding. Definitely a bigger, bolder sound which you might find better or worse.
well, genre versatility is pretty important to be so i tend to prefer more uncolored/neutral iem's for the most part. more sub bass would be appreciated. all in all though, given the trickiness with the fit issues with kef m200's i'm not sure if they're worth the upgrade. the new brainwavz sound interesting but i'm generally not a fan of bright earphones so i'm not sure.