I did an upgrade on a laptop that I use for such experimenting. I was very surprised at how little trouble I had. The OS install would not move forward until I removed AVAST. I didn't have too much junk on the laptop to start with which might have helped.
All my other stuff was preserved, My Documents, My Pictures, etc. Connecting to the net was painless; all my programs work. Network Meter gadget ver. 8.1 works but doesn't display internal IP address like it used to. SuperAntiSpyware, MBAM function properly and I got MSE in place of AVAST.
As soon as I get more familiar with the OS, I'll try a clean install, but was pretty pleased that I didn't have to spend hours re-configuring a laptop if I didn't have to. The OS is pretty intuitive and although it took sometime to download, "prepare files", a couple of reboots, and do settings, it was 2 hours or so of impatience. Navigating around takes a bit of getting used to. I had no "screens of death" in any color and the fish did sit on my desktop for quite awhile before anything happened.
I know the clean install is recommended by many, but I thought I'd give this a try......and fortunately it worked out.