It's an undertaking to go back and redevelop a site if there is no business reason. If you develop sites for a living then I would wait until your customers need a redesign or have a business need for it. Also I would start to look at HTML5/CSS3 and responsive design for your newer customers moving forward.
I am managing a pretty good-sized number of sites and, to the extent I need video, I have converted just those sites to HTML5.
New websites start out in HTML5.
Fact is, the last standards for the worldwide web were written in 1999. And those standards tend to work in all browsers, save Microsoft browsers, which aren't consistent even within their own standard. So a website built using XHTML 1.1 transitional or HTML4 is perfectly valid and supportable—except for video.
With respect to video, switching a website from XHTML or HTML4 to HTML5 is really simple. I have tended to not rewrite the site and simply changed the declaration, included the HTML5 Shiv, inserted the video tag and the video link and moved on. Everything works, everything is fine and everything degrades nicely in the non-compliant Microsoft browsers.
Within the <video> declaration, I encourage everyone who cannot see the video to upgrade to a modern browser, like Firefox, Chrome or Safari.
Personally I have NOT moved to html5 yet although I know most of the new tags, I dont really give a hoot if you use:
<footer></footer> or <div id="footer"></div>
Who cares really, both work, both do a job. I'm not into all this semantic crap to that extent. Most clients just want the site to 'visually' work x-brower - they don't give a toss about the code whether it validates or not or you use images to create shadows/gradients or css3.
I'm not sure my opinion will change until IE8 is completely dead, its not at the moment.
Actually, I find <section> <article> and <aside> very useful. But that's me. If you're not using <canvas> or <video> with respect to what you are doing, HTML5 is probably not necessary, as Osgood is correct. The semantic naming of a div may not be any different in XHTML or HTML4 than the new semantic conventions of HTML5.
HTML5 is not a standard yet. It is just a proposed standard. Thus, there is no requirement that we use it. But since it's a proposal that is pretty complete, now might be the time to ease into it.