If you did a fresh install, it would wipe everything for sure. But doing the Upgrade really is just an upgrade. Never do anything without backing your data up, but after running the Upgrade, I still had nearly all my programs intact, and all of my files were preserved. The Upgrade process tries to keep your "current state" as close to what you already have as possible. All my desktop icons and files were in the places I expected them to be, and I didn't have to reinstall anything (again, that's gonna vary from person to person depending on what you have). I only reinstalled Destiny because I wanted to make sure it was going to run on a clean fresh install of that.