While Microsoft's latest Windows Phone 8 update is slowly rolling out to existing handsets, the company is prepping new changes that will be made available by the end of the year. Sources familiar with Microsoft's Windows Phone plans have revealed to The Verge that the software maker is currently testing a General Distribution 3 (GDR3) update. The update is designed for new hardware initially, and will provide a rotation lock feature, UI changes to Live Tiles, and a driving mode option that's designed for in-car use.

The rotation lock, or "autorotate screen" as it will be known, is simply an option in settings that will disable the automatic rotation in Windows Phone. Microsoft's mobile OS has lacked this basic feature, and we understand it's being added primarily because 5- and 6-inch Windows Phones will hit the market soon. The Windows Phone UI will also alter slightly, with some changes to the network status icons, and updates to the Live Tiles for new 1080p handsets. We're told that Microsoft is also planning to sync text messages from Windows Phones to PCs and the web. The feature might not make it in time for the GDR3 release as it's still being finalized, but we understand it's designed to let Windows Phone users view text messages on the web and on PCs, without having to view them on the handset. Some GDR3 features were revealed in a Reddit post this week.

The update is coming with new devices first

We're told that Microsoft is aiming to finalize the GDR3 update in mid-September in time for a new large-screen Nokia device announcement. The update will be available on new devices initially, before it rolls out to existing devices before the end of the year. Microsoft is preparing a larger Windows Phone 8.1 "Blue" update for early 2014 that's expected to include a notification center and multitasking improvements.