It's not just a restyle though: it's in most cases a complete rewrite. At the very least in almost all cases it is a very new UI framework compared to anything the control panel was previously written in (WinRT/XAML versus organically grown madness across mixtures of MFC/ATL/Win32-by-hand/other forms of macro soup). But if Old New Thing and other Windows blogs make anything clear, it's also a ton of new testing, because Microsoft has no idea ever what sort of changes might domino into some user's beloved fifteen year old application that no one realized instead of using the right API for its job was instead scraping the pixels of the control panel for the right shade of purple.