Why did the Federal Reserve change the data about USA GDP?
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.