Windows 7 Anytime Upgrade

The System Properties page shows the currently installed version (highlighted in yellow) and includes a link to "get more features with a new edition of Windows 7."
For full details, see Microsoft prepares Windows Anytime Upgrade, v2

This Windows Anytime Upgrade screen offers two options. The top link (not yet live) will allow you to purchase an upgrade key directly from Microsoft. The bottom option lets you enter an upgrade key you already purchased, presumably from a Microsoft partner.
For full details, see Microsoft prepares Windows Anytime Upgrade, v2

The upgrade key uses the same 25-chaaracter alphanumeric format as the keys used to install and activate a new copy of Windows Vista or Windows 7.
For full details, see Microsoft prepares Windows Anytime Upgrade, v2

When I tested the upgrade process using a Windows 7 Ultimate product key, the process took just under 10 minutes. I never had to supply a DVD, and I didn't have to restart at the end.
For full details, see Microsoft prepares Windows Anytime Upgrade, v2

This status screen (and a similarly upgraded System Properties page) indicate that the upgrade was successful.
For full details, see Microsoft prepares Windows Anytime Upgrade, v2