Now, if you want to talk about big changes, it'd be when I went 250. The two bikes I have now, the 120 and my Husky, mark the transition. There wasn't anything in between the minibike-sized 120 and the 42-inch-seat-height and 220-pound Husky. In one day, I went from four speeds topping out at 45, to six speeds topping out at 85. It was sort of scary at first, because it felt like I was sitting on a bloody skyscraper, but once I got moving I noticed that it wasn't any more wild than the 120. In fact, I've never had the 250 scare me with its power, while the 120 has given me a number of surprise wheelies out of nowhere.
I guess there's not really a lesson in this little story, but it was certainly a big change. Check the size difference.
(This was before we "fixed" the 250, so that's why it looks so rough.)