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We currently have great SEO utilizing HTML4 entities, we are considering utilizing an HTML5 document type but are concerned about the SEO implications.
We do not want to risk losing our SEO by simply updating our front-end implementation.
Questions
Google doesn't favour HTML5 over HTML4 per se. However, HTML5 does allow more semantic mark-up, which will make it easier for Google to figure out what's what on any given page. This allows Google to be more precise when it comes to ranking what is and what isn't important on a page.
For example, the nav element indicates very clearly to Google (or any bot, for that matter) that the links contained within are the "main" links for the various sections on a website.
I've never heard of a page dropping in ranking after adopting HTML5 doctype, though I have seen and heard of more than a few that have risen in ranking (mostly for branded keywords) after making the change. Though I've always (personally) attributed that to the more semantic nature of HTML5 than any preference of Google's.
Google doesn't treat HTML5 differently than HTML4 - there is no inherent advantage to using HTML5 elements on your pages (and no disadvantage either). That said, often HTML5 pages are coded cleaner, frequently they smaller (better for users) and use valid code (better for users / cross-device compatibility).
That said, anytime you make significant changes on your website (such as a redesign - be it in HTML5 or HTML4) you will almost certainly see fluctuations in how search engines present your website. If you change the URL structure in addition to just the layout, this is even more so. That's completely normal, and provided your redesigned site isn't broken, it'll generally return over time (sometimes that's a few days, other times it can be a month or longer).
Google has never officially said that DOCTYPE markup contributes towards negative or positive SEO and from the millions that have already converted to HTML5 there has been little or no reports as far as I know that has had negative SEO.
HTML5 Isn't new!
While most will consider HTML5 pretty new its actually some what aged as it has already entered RC phrase and they are already contributing to HTML5.1 Spec which is due in 2015/2016.
Strict Standards
You can read W3C nightly releases which covers most HTML5 and HTML5.1 markups as well as a favorite site of mine html5doctor.
