I'm in the same boat, I purchased the 400s back in August and now could not resist trying the 400i at the Black Friday price.
I received the 400i yesterday and I've only had a few hours with them but they are fantastic. I'm not going to say they are all-around better than the 400s.
The 400s is the king of smoothness. They are probably the least fatiguing headphone I've ever used. That smoothness comes at the cost of some treble crispness and rolled off bass, not to mention a bit more of a homogenized sound from top to bottom.
The 400i has more detail overall, nice deep bottom and overall greater clarity. Theses enhancements to the 400i sound character create a bit of, I hate to use the word, harshness. When I use the term harshness I'm not implying something negative as the 400i are so much better sounding than most headphones I've experienced and that harshness in this context is in relation to a higher order of magnitude in terms of headphone quality.
At the moment I plan to keep both pairs as they do have a significantly different sound presentation. I'll will wait until at least the used market recovers from this sale before I will consider selling the 400s. I need to let the 400i break in a bit more before I make a full judgement too.