Do we need to retest intranet applications that are designed for Internet Explorer 7 or work with Internet Explorer 8?
Not necessarily. Internet Explorer 9 was designed to have a high degree of compatibility with Internet Explorer 8. In particular, Compatibility View mode is on by default for intranet websites in Internet Explorer 8, and that setting works the same way in Internet Explorer 9. Compatibility View mode allows websites that were designed for Windows Internet Explorer 7 to work properly in Internet Explorer 8 and Internet Explorer 9.