Others may have better answers for you. I don't know when the cabin was first extended, but I've seen used models back to at least 2006 with the EC cabin. So the change had to be before then. I don't know what the hull change was or when it happened. Maybe about the same time?
As for the 2012 versus 2013, at the Seattle boat show, I got the impression there were no big changes on the 21 EC, but perhaps some minor ones. The upgraded upholstery is now standard on all models. There may be newer versions of the electronics packages since those change as Garmin brings out new stuff. A few things have been changed on the interior trim on the other boats, and may differ on the 21 EC too (the dash on the others, for example, is not wood now, but is stitched brown vinyl -- handsome and seems practical). Can't think of anything else. If you can get to one of the big boat shows, you may find that the promotional factory boatshow pricing on a 2013 will be competitive with a 2012. But dealers may be motivated on the 2012, and a 2012 would be ready for delivery now -- on the 2013, unless one is already at the dealer, you could be looking at a bit of a wait.
Hope that helps a little.
Gini