My concern is, my client is already number 2 on the search engine ranking for the main keyword of his particular field and I don't want to see him lose any more ground. He wants to not only future proof his website with html5, but also add new content. With that being said, he was number one on the search results for a long time and is now number two. Furthermore,  I fell like he's quickly losing ground to other websites. Providing that all the keywords and links are taking care of, will a complete update hurt or help his rankings.
How will a website update - from HTML 4.01 to HTML5 - affect page ranking?
html5;seo