to chápu jako možnost "přepnutí" (bez reinstalace) po dokoupení, např mezi verzí např Home na Ultimate a tak. /což jde, stačí zadat do windows jinej klíč, ale jakási miniinstalace se provede/.


jinak na tom předch. odkazu
64-bit DVD
If you bought Windows Vista Home Basic, Home Premium, or Business as a retail, packaged product, and you would like a DVD of 64-bit software media to install on your PC, please choose your language:
dál je pak možnost objednání inst. média
Note: 64-bit media is included in the box with Windows Vista Ultimate.

Here's a FAQ with more details about 32- and 64-bit Windows.