It will be interesting to see how the OS installs over itself now that it is being distributed through the app store. I wonder if they will create a new partition with the installer and reboot into it.
On Unix systems you can replace files on disk without issues because of the way it does reference counting to the underlying file system. Unlike Windows where this is not possible. I am better there is still one single reboot to start up the new kernel and whatnot. Although I can't find any details on it.
The reason why you can replace executables on Unix is because unix loads the entire file before executing where as Windows does not need to do this, hence executables are opened with out FILE_SHARE_DELETE which would allow you to delete the file while it's open on Windows. If you create a file and open it with FILE_SHARE_DELETE you can delete it while it's in use.
This has absolutely nothing to do with the underlying file system format and everything to do file semantics as documented in the Win32 API.
http://msdn.microsoft.com/en-us/library/aa363858(v=vs.85).as...