According to the Japanese site MyDrivers.comMicrosoft sent on December 10 to its OEM partners ( Original Equipment Manufacturer ), a draft of the Windows 7 Upgrade plan, aiming at a Feedback from them.

The requirements for a user to be able to make the big update are:

• 1. The most obvious of all: The user needs to have an original copy of Windows Vista installed on the machine;
• 2. The PC shall contain a valid safety certificate (AOC);
• 3. Only the following versions of Windows Vista will be available for upgrade: Windows Vista Home Premium, Windows Vista Business and Windows Vista Ultimate. That is, if you own any other version of the system (Windows Vista Home Basic, Windows Vista Starter or Windows XP), if f*gave will not be entitled to upgrade;
• 4. The scheme does not apply to corporate users of small, medium or large enterprises;
• 5. The update will be made in the following criteria:
• * Windows Vista Home Premium -> Windows 7 Home Premium;
  * Windows Vista Business -> Windows 7 Professional;
  * Windows Vista Ultimate -> Windows 7 Ultimate.

Now you just wait for more information, something might change until the official announcement.

Source: PC Guide