Why did the Federal Reserve change the data about USA GDP? This is the data published on 20 March 2013 FOMC meeting:
(http://www.federalreserve.gov/monetarypolicy/fomcprojtabl20130320.htm)

20 March 2013,
Change in real GDP (actual):


year 2008: -3.3%
year 2009: -0.1%
year 2010: +2.4%
year 2011: +2.0%
year 2012: +1.6%



This is the data published on 18 September 2013 FOMC meeting:
(http://www.federalreserve.gov/monetarypolicy/fomcprojtabl20130918.htm)

18 September 2013,
Change in real GDP (actual):


year 2008: -2.8%
year 2009: -0.2%
year 2010: +2.8%
year 2011: +2.0%
year 2012: +2.0%



As you can see, the GDP data for year 2008, 2009, 2010 and 2012 that published on 20 March 2013, and the data that published on 18 September 2013 is different.
Why did the Fed change the data?
This happens all the time and is normal. It's due to measurement error. It is hard to measure these things in real time and as time goes by more data comes in, which allows measurement of GDP and other macroeconomic variables to be better so they are revised. Sometimes the revisions can be quite large. Typically the latest data is also the least reliable.

For some advanced information on how such measurements are made, the issue of measurement error and a discussion of how to improve estimates see this paper

For a discussion on the issues these revisions of GDP data and measurement error pose for monetary policy, see the work of Orphanides, for example here and here. For his discussion on the reliability of GDP estimates in real time see this paper.