I have installed 8 clean and as upgrades on my client's and my computers.
It seems, if the 7 install is good, clean, happy,... then 8 works fine, and then 8.1, and then 8.1.1.
Before the upgrade I always create a disk image. I do a complete disk clean, including deleting all but the most recent system restore point. I manually delete all the temp files disk-clean does not delete. Stop the swap and hibernate files: both are related to amount of ram. Don't need to wait to image them. (After restart verify the pagefile is gone. If not delete.)
For the music computers, I make the OS partition/disk be as clean and spare as possible. I have the download folder, and music and pictures and video files on a separate disk. I also have dimension pro,... samples on a separate disk, as are the plugins, and sonar sample files, and tutorials,... The disk the OS is on is partitioned to keep the OS disk image small. The other partition on the disk is for storage of installation files, and audio file backup, but not of the disk images.
A couple of plugs wanted new authorization (I think Ozone, and Sony CD architect and Noise Reduction.. Went quick and easy. I did reinstall the drivers for the audio interface just to be sure.
I have/do create an image for each major OS change. Does not take much time using an USB 3 external drive. The primary music computer has a 3 Terabyte disk, so I send the image to it.
I keep each image for a bit, so I move the backup folder to another folder and change the name. If I need to restore, I copy the folder to the 'win image backup' folder and change the name to the original name. Otherwise windows will not restore. If the image remains in the image folder with the original name windows will overwrite it, if the computer name is the same.