Funny. never have done a clean upgrade. Unheard of in Windows world. But since 06 (Mac Pro initial) I have been able to do in-place upgrades first on the mac pro, then when I bought a Mac Mini, I did a time machine restore to the new box (No issues at all, it just worked) then more in-place upgrades now up to 10.9 Pretty amazing in my book. I still have my a windows PC (one I built last year for gaming) But going from windows 7 to windows 8.1 I actually had to reinstall all my apps etc.. since Windows does not seem to support such upgrades. When I upgraded from 7 to 8.1 I was warned that all my applications would have to be reinstalled. You would think after all these years Microsoft would get it right.