HTML5 accepts mostly both XHTML (strict) and HTML4 (strict) ways of coding, so you should not be worry. In other words, HTML5 is almost fully back compatible with the strict versions of HTML4 and XHTML. With some exceptions, of course, but not so important.
HTML5 is not a new language. It only comes with new features, but it keeps also the vast majority of the older (HTML4, XHYML) ones, so don't worry, use HTM5 Doctype. Later, if you need, you will be able to use those new features.