Ok, it's a bit of a cheap shot, but check out the comparison of upgrade methods between Microsoft Windows 7 and Apple Snow Leopard. In the Microsoft world, upgrading is hard. Of course, I think it's pretty obvious that if you have only one version of your software (take this as a good thing or a bad thing I guess), the upgrade path will be light-years easier compared to having 3 or 5 different versions to dance from/to).