Microsoft is not supporting upgrades of any kind to Windows Phone 8, not from the Nokia Lumia 900, and not from any other Windows Phone 7.x handset. However, Microsoft is throwing a bone to existing users in the form of Windows Phone 7.8—Get it? 7.8 … 7 to 8?—which will provide only the barest of Windows Phone 8-like features. In fact, it will provide just three.
Start screen. The Windows Phone 7.8 Starts screen will be changed to look like that of Windows Phone 8. It will fill the entry screen with tiles, and not leave a strange “white space” gutter on the right side, as does Windows Phone 7.x. (This was apparently the source of some great number of user complaints, the theme being that Windows Phone doesn’t completely take advantage of its onscreen real estate. Commoners.)
Tile customization. Windows Phone 7.8 will now support three tile sizes, just like Windows Phone 8, and not two like the current version. And apps can be written to support any of the three sizes, giving the user the ability to choose between them (as is done with Windows 8’s live tiles). In addition to the current Small (square) and Large (rectangular) sizes, Windows Phone 7.8 and 8 will also support a new Smaller size, which is a fingertip-sized square that is one-fourth the size of a small Tile. Mix and match as you please.
Windows Update. Windows Phone 7.8 will be delivered directly to all Windows Phone users, bypassing the carriers. You will be able to download and install Windows Phone 7.8 over Wi-Fi, at home or wherever else, and install this update. This type of updating will be made available in Windows Phone 8, as well, as given my long, loud, and lengthy complaining about the lack of such updating in Windows Phone 7.x, you won’t be surprised, maybe, to discover that Microsoft jokingly calls this “the Paul Thurrott feature” internally.
Want to learn more about Windows Phone 8? I've got the full scoop in Windows Phone 8 Unveiled.