If it has to happen anyway regardless (take this situation for example) it's better to throw money at them to let them hire top notch people to continue where the others left off.
Then the top notch people have to learn everything that's happening, which is a lot of re-education. Read the book, "The Mythical Man Month", it's one of the foundation books on project management and that's where they discuss why adding peopel to a project rarely speeds it up. It's better, in general, for them to suck it up and delay the project or drop features.
Well Vista has been delayed a while but I doubt new project managers will be touching much of the code in this particular instance.
If the way Windows comments code hasn't changed from Windows 2000 (Not that I saw it
) then I don't think there will be a problem with commenting at all and most of the programmers who program low enough in the kernel usually know what they're doing.
In general I think people implementing higher things like applications are the ones who tend to get replaced a lot which results in messy code for Windows applications. Of course with WinFX including XAML application development time is cut probably in half.