It is a transition that will just slowly happen, it's not like much has been taken away, its more so the case that features have been added. Embedding video and song directly for example, mapping vector images etc... These are all features that make web design a little smarter.
Some of these features are not optimised for each browser, however there is a way around it to make them function in most. The same as when in CSS we pick a font, then comma separate following fonts for any browsers that do not display the first choice. HTML 5 features work similarly.
From looking over the new standards it doesn't seem very different at all, an easier doctype declaration, and a few new functions, that may or may not be utilised in your websites. If you have a fully fnctioning website written in HTML 4 and you changed the doctype to conform to HTML5 standards, providing you didn't use any of the brand new features, your website would work the same way in all browsers as it already does. Whereas once you start drawing vector images and embedding direct video, you may run into some browser issues. That is the only way I can explain it.
To summarize, go ahead, get used to the new standards and experiment. If something doesn't work the same in all browsers, find a way around it, or take it out for another few months until the majority of browsers do. Hope this helps.