If you have the budget, do some sort of drive image/bare metal restore capable backup after each major step - Acronis TrueImage, Paragon Disk Manager, Clonezilla, FSArchiver, or even a GParted (Parted Magic, and I think System Rescue CD) or DD partition copy to another drive.

These allow you to pop in a brand new drive (or overwrite your current drive if the upgrade breaks it completely) and simply restore to exactly where you were at the time of the backup, instead of spending days rebuilding from scratch.

Note that for Windows Server, you cannot use the desktop versions of Acronis or Paragon - you must spend a couple hundred to several hundred dollars on the server versions. The others listed are open source.