Is there a tool from Amazon or Microsoft to automatically upgrade an ec2 instance's sql server 2008 to 2012? Or do I have to create a new instance and then import all the databases?
One of the joys of EC2 is that you can create instances so easily, for the tiny cost of a few hours of runtime. I've had so many system upgrades (win/mac/unix) go wrong that I would recommend doing a test run on a different instance no matter what.
But to answer your question, I know of no Amazon tools for updating an instance from 08 to 12. Microsoft's website contains enough Notes and Cautions that I would be careful with any tools from them as well.
That web page does say specifically: