Is there a keyboard shortcut to change tabs in Notification Center?
I hate these kinds of answers, but there doesn't appear to be one.

Another question points out that you can use some kinda-hacky Applescripts to work around the limitation: Does Notification Center have keyboard shortcuts?

I was toying around this, and it seems the key combos that notification center responds to depends on the active widgets. If you're in notification mode, you can reliably jump back to widgets by hitting TabTabEnter, but on Widgets, I find that the calculator tends to monopolize all keyboard input and prevent any tab jumping from working.