I sold my 300 f/2.8L I IS and 400 f/2.8L I IS lenses (because I don't really need the 300 anymore) and bought a 400 f/2.8L II IS for sports all next year.
May I ask what was your rationale? I would never have gone from the 400 f/2.8L mrk I to II myself.
I did this with the 300mm f/2.8 L and while its nice in many ways - weight, handling, faster IS etc - it was really a luxury upgrade as IQ differences are so negligible that I doubt anyone who has not owned both would be able to spot the difference.
I don't regret my upgrade as I can easily afford it and splash a little extra on my hobby. But for a working tool I would not even have considered it.
That's just it. It's going to be a working tool for me, and I could not handle the excess weight running around at football games and track meets anymore. The was my first consideration. The second was that I don't need my 300 anymore. I want to use the 400 and 70-200 combo. I had to crop too many times with a 300 lens on FF. Those two things coupled together made the price worth it to me.
As for Northstar's question, I absolutely considered the 200-400 lens. However, many of my venues require f/2.8 to keep the ISO down low enough (some places just to get to 5000) so I didn't purchase that. I do agree, however, for daytime/sufficient light, it could potentially eliminate even using a 2nd camera and shorter lens if you can get back away from the end zone and can shoot at 200. That would be an excellent point. But there are just too many night games/events for me right now.
Again, thanks everyone for the comments/contributions. I will be able to produce photos the weekend of 9/6-9/7.