Microsoft will stop supporting Windows Phone 7.8 and 8 by the second half of 2014.
The company posted a table on its site outlining its support plans for the different versions of its OS, with Windows Phone 8 support ending on 8 July 2014 and WP7.8 getting until 9 September of the same year.
The timetable means each OS gets 18 months of support from launch. In that timeframe, most users of phones with the OS will be eligible for a handset upgrade.
Microsoft launched a new SDK for Windows Phone 7.8 in January but has provided little detail on when the platform will be available to existing Windows Phone device owners.
The upgrade to the 7.5 Mango version of Windows Phone is aimed at making the OS more closely resemble Windows Phone 8 with the addition of Live Tiles. However, it is only available on new devices.