We do know that Microsoft has confirmed the Windows Phone 7.8 update for early 2013, but the update has already appeared on the Nokia’s servers for several devices. The WP 7.8 update first appeared for the Nokia Lumia 800 and the Nokia Lumia 510. It was also discovered for the Nokia Lumia 710, Nokia Lumia 610 and even the former flagship the Nokia Lumia 900.Unfortunately the update has still not been made available for the Nokia Lumia 900 for AT&T.
The most (and probably only) significant feature on Windows Phone 7.8 is its new Start Screen, which features the same three sizes offered in WP 8 devices. Other features include 10 new theme and accent colors, Bing Picture of the Day on the lock screen and security updates.
A user who downloaded the update for their Lumia 900 said that it doesn’t include the new Nokia apps like the ringtone maker or Bluetooth share which was available for certain variants of the Lumia 800. However there is some speculation that Nokia will add new features including updates to the Lumia 900 camera, after the WP 7.8 update is released.
If you own the International Lumia 900 and feel adventurous, you can check out our guide on how to install the Windows Phone 7.8 update on your Nokia Lumia device from Navifirm.
If you have a Lumia 900 on AT&T we wouldn’t recommend installing the update from the international version as you may loose your LTE connectivity or worse. Since the year is almost done, it wont be long before the WP 7.8 update is officially released. Its better to wait for it to arrive than risk bricking your device.
[source] WPCentral