It’s a bit more than that, you’ll have to enable it, but you’ll also have to go through all of your files and replace the old Genesis Code with the new Genesis Code. StudioPress replaced their old markup code with ID’s, Classes, and functions that were more suitable for HTML5. So that meant everyone that created child themes for the Genesis Framework must update and replace there code with the new versions.
Take a look here if you haven’t already – http://appfinite.com/topic/legacy-1-3-for-genesis-2-0-html5-is-now-available/
There are also some sites out there that automatically go through and replace your code in the style.css files. Try out the link above first and that should point you in the right direction of what you’ll need to do.