Microsoft is trying to make it very worth your while to upgrade to Windows 8:
…Starting at general availability, if your PC is running Windows XP, Windows Vista, or Windows 7 you will qualify to download an upgrade to Windows 8 Pro for just $39.99 in 131 markets. And if you want, you can add Windows Media Center for free through the “add features” option within Windows 8 Pro after your upgrade.
And now, the “catch”:
I am familiar with some upgrading scenarios and procedures that did not end well, so I warn you to CREATE THE BOOTABLE USB AND/OR DVD!!!
After your download finishes, you can choose to proceed with the upgrade (“Install now”) or install later either from your desktop or by creating your own media. If you choose to create your own media, you will be able to create your own bootable USB or .ISO file which can be burned onto a DVD for upgrade and backup purposes. If you prefer, you also have the option of purchasing a backup DVD for $15 plus shipping and handling.
In the past, some decided to upgrade their operating system via a download from the internet. The DID NOT create installation media (DVDs). The installation procedure failed, and everything on their hard drive was lost: including their data, programs, and the operating system (Windows).