In the next couple of days we'll be switching the default analyzer (the analyzer that is used if you don't explicitly specify the analysis_version) to version 3 of the analyzer. This analyzer is more robust and in general gives better results. This change will affect you in the following ways:
If you try to obtain analysis results for tracks that you've previously analyzed with version 1 of the analyzer you'll find that the results are unavailable until you reanalyze the tracks.
The results may not be exactly the same to version 1. In particular we've improved some of the beat tracking algorithms. Remixes may sound different, and overall should be more accurate.
You can maintain backwards compatibility by setting the 'analysis_version' parameter to '1' for the track APIs.