I did what you are talking about. I love my USA made bike and the lefty. I went from an 80mm Headshok to a 110mm Lefty. As for as the geometry, there will be very little change. I am not capiable of giving you a bunch of numbers and angles and waht not but I can tell you what I have experienced. For my bike, it only made it better with improved turning and feeling. I would not go back to the headshok after my experiences with the lefty. The headshok is a great fork but the lefty is just so much better.
I have some photos for you to see below of before and after.
Before:
After: