Assuming you ran 10% sag on the 80mm (8mm) and will run 25% sag on the 120mm (30mm), you are really raising your front end 18mm (120-30=90mm vs. 80-8=72mm). Not as large a difference as you might first think. However, you will (most definitely or theoretically - take you pick) feel the difference when climbing (hard to keep front end behaving), on tight switchback (front will tend to go wide), and on fast downhills (you'll love it). But I'm not an engineer, so what do I know. You already have it, so throw it on an go ride it. I love experimenting with Frankenbike stuff like that! Gotta post back and let us know how it worked.