Windows 10 has the ability to rollback to the previous version of operating system you were using. But it is still advised to keep a backup of your system just for your safety. As far as Windows XP or other versions, Microsoft has provided trial copies of Windows; you can download and install to give it a test run before committing to. Recently, Microsoft provided a 90 day trial for both Windows 7 and Windows 8.1, you can download as a .ISO image and then burn to a blank DVD or you can also create a bootable copy on a USB thumbdrive and can then install on a blank partition. The same will likely be available for Windows10 as well. Actually, you can set up a partition now and then install the Windows 10 Insider Preview on it and then you can also evaluate it throughout the year without disturbing the commercial commitment. If you find the features you like, delete the partition and then upgrade the commercial copy when you are ready.