The 42U HTML5 Upgrade prepares your older site for HTML5 and allows you to start using HTML5 structure tags right now.
Anyone with an older site template that is not written to HTML5 standards.
Probably not. We deployed this on over 350 sites that we host and didn't see a single problem.
From the modernizr docs:
That means you can develop for the future, with the real API, and only load your compatibility polyfills on browsers that do not support that API or feature.
And good news for you, there is a polyfill for nearly every HTML5 feature that Modernizr detects. Yup. So in most cases, you can use a HTML5 or CSS3 feature and be able to replicate it in non-supporting browsers. Yes, not only can you use HTML5 today, but you can use it in the past, too!
See https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills for an extensive list of available polyfills.