Microsoft is revising a program that allows Windows Vista customers to upgrade from a basic version of its OS to one of the premium editions.

The program, called Windows Anytime Upgrade, was designed to make it easier for customers to upgrade to a more expensive version of Vista by allowing them to purchase a digital "product key" and download it from the Web.

At the time that they purchased their OSes, customers received a DVD containing several versions of Windows Vista, including the premium editions. Downloading the new product key allowed them to unlock a higher-end version from the disc.

Microsoft has now decided to stop distributing product keys online, it said on Thursday. Starting Feb. 20, customers who want to upgrade will have to buy a Windows Anytime Upgrade kit from a local retailer, or via postal mail from Microsoft. The kit will include a Vista DVD and a product key for the version they are upgrading to.

Microsoft said it was making the change based on feedback from users. The new system means customers won't have to worry if they lost their original Vista DVD and now want to upgrade.

However, it also means that people will now have to visit their local retailer or contact Microsoft via post (an option not available in Japan).

The price to upgrade will be the same as it was under the previous system, which was $79, according to Microsoft.

"Customers have told us that this change will streamline the Windows Anytime Upgrade experience, and that they would prefer to use the DVD and a standard 25-character product key to perform the upgrade -- rather than the combination of the DVD and digital key," Microsoft said.

More information about Windows Anytime Update can be found on Microsoft's Web site.