That should only happen if the installer finds a problem with your current installation. Also be aware that if you're on Playstation you will 100% still have the copy function which copies the entire game before performing the update. If you're on a PS4 and still using the mechanical internal drive prepare for a wait for the copying. If your D2 is installed on an SSD though it shouldn't take much time to perform the copy. I've never had a D2 Playstation update take longer than 20 minutes from start to finish.