This is by design. Changing photos to Process 2012 will cause your photos to look different. This has been discussed in many places. I can't answer "why" Adobe did this. Most people are delighted with Process 2012 -- no, not the direct conversion with no further edits, as you are experiencing, but they are delighted with using Process 2012 to re-edit their older photos or to edit their new photos.
Since Lightroom is non-destructive, you can always go back to previous states. You can do this image by image (yeah, I know) in the history panel. Or by setting the Process Version on one photo back to Process 2010 and then synchronizing all the other photos back to Process 2010. Or, if you have a backup of your catalog file, it's easy to go back to the Process 2010 appearance of the photos.