Making the switch to OneNote 2010 from OneNote 2007 is pretty painless. It opens all of your existing notebooks and runs them in Compatibility Mode, which ensures you can still share them with users who haven't upgraded yet. But it does mean you won't be able to use new features such as multi-level subpages, versioning, or linked notes in them.
If you do have a notebook that you want to update, right click its notebook icon on the left side, select Properties, and click the Convert to 2010 button.
That was easy! But if you look at OneNote, something still seems off. Oh, wait, I know: The Ribbon is missing! Actually, it's just disabled by default. To bring it back, simply click the down arrow at the top right next to the Help question mark.
And here you have it, the Ribbon:
Ah, that's much better! Now to get some work done.