Go for it. I got a 2006 version to get me through MS1 (bought it the summer before med school, scanned through it before MS1, and used it during).
A few weeks ago (I just finished first year) I bought the 2011 which I will use for MS2 and the real deal.
To answer your question, they do change and add a considerable amount of things, which I presume is more significant for boards than for the classes themselves.
But...you'll want a fresh one for MSII in my opinion. During MSI you tend to annotate a lot of things, some of which is useful, and a lot of which is not useful. So it helps to start with a clean slate, go through your old FA and keep the useful things to annotate into the new FA.
I am super glad I did not spend 40$ on the 2009 or 2010 FA only to buy the 2011 for MSII (some people will go all out and get the 2012 for boards). I got mine via better world books for 4$