Hi Jay,
You wrote:
I've just upgraded my OS and my iWork.
Look in a folder called iWork '09 inside your Applications folder. You may still have Numbers 2 (Numbers '09) on your machine.
Wayne Contello has written a User Tip for working with the old and new versions of iWork.
https://discussions.apple.com/docs/DOC-6991
The trick is to drag icons for both versions to your Dock. Right click or control click on each and Options > Keep in Dock. That puts you in charge of which version you launch.
Here is a screen shot of both Numbers icons on my Dock.
Numbers 2.3 (Numbers '09) on the left with a three column graph.
Numbers 3.2 on the right with a four column graph.

Both versions will run happily together. Just be sure which version is opening your document.
Instead of double clicking the document, run whichever version of Numbers/Pages/Keynote suits and use File > Open.
Regards,
Ian.