Naturally, they did this by default, and then documented a mind-bogglingly complex workaround (KB 938810) rather than providing a user interface for adjusting it, or even a set of awkward "Do you really want to do this?" dialog boxes to click through. And, of course, because these are, after all, old file formats, many users will encounter the problem only months or years after the software change, while groping around in dusty and now-inaccessible archives.
One of the better aspects of Office is its extensive compatibility mechanisms for old file formats. At least the support isn't completely gone—it's just really hard to use. Security is important, but there are better ways to fulfill this goal.
This was also covered by the Windows Secrets newsletter, although I can't find a story URL for it."