#1. "RE: Puzzled with LR4 Relating to Previously Processed LR3 Images" In response to In response to 0
First, when I view the before/after I don't get the photo split? Look on the left above your navigator - make sure you have "Fit" selected.
The PV2012 sliders are different. There is no exact copy of a 2010 slider to a 2012, so it will likely affect more than one 2012 slider meaning that the result may not match your original (2010) version 100%. In most cases I am seeing a bit more contrast, and in most cases I like what I see, so I don't have a problem with it. However, if I don't like what I see, the before and after give me an idea of what I might want to adjust in the 2012 sliders to get back to what I might like better.
I checked many of my high-rated images (8000 total, I maybe checked 50 randomly) and I liked all of them -- so I just converted all 8000 to PV2012. However, knowing I did that, if/when I go to print something I will take a quick look to see if its still to my liking. But that is just what "I" decided to do and may not work for everyone.