It was the box that changed from silver to black. They havent done anything with the drivers for a long time. I know head-fi was all OMG the 650's have changed but at least one guy revisited his post after putting a good amount of use through them:
http://www.head-fi.org/t/197776/sennheiser-hd650-impressions-thread/31440#post_12192490
So i dunno, not convinced.
I've owned numerous HD650's from the black drivers to the black box the differences I found were..
Black drivers: Darker, warmer with a more lush presentation.
Silver drivers silver box: Warm but less so than the black driver version, cleaner sounding, less murky same everywhere else.
Black box version with silver drivers: bass is less woolly than the others but is equally as deep, cleaner sounding. Because the bass sounds less elevated the little 9k peak sounds like it has more energy.
I think the changes are down to the ear pad material, the newer versions are a bit more dense feeling. Also possible changes to the materials in the driver but I'm just speculating.
All 3 sound exactly the same in extension, resolution but the older versions sounded more lush, but a bit more sleepy sounding. The silver box and silver drivers and the current black box are probably the same and the materials just need time to settle but there is a difference in sound especially fresh out the box for quite a while.
I do def hear less mid bass with the current black box versions although one I received was very bright and brittle to the point I thought they put HD600 drivers in my 650's but I noticed the ear pads were very thick on that one so maybe again pad material.
I don't think there's any point worrying about which one you get, I wanted a silver box version but because one of my black box version seemed to have started to sound similar I just didn't bother. If you want a headphone that has tight bass then you should ether get a closed back headphone or look into the larger planar magnetics but neither will have the resolution and mid range voicing the 650 has.