Once the happy part (or major, as some music snobs would call it) play it in a different tempo. I don't agree with faulty that it should go slower though. I think you should start in the same tempo, and then go faster and faster, as you'd speak, full of excitement!Imagine that you hear really good news, and that you tell everyone about it. You'd start in a normal speed, and end up in uncontrolled ecstasy. That's the true way of playing Mozart. That's the true way of speaking it, and therefore the way of playing piano!