Microsoft is planning to distribute its Windows Phone 7.5 update to existing AT&T HTC HD7s devices on Tuesday, WinRumors has learned.

AT&T has signed off on its HTC HD7S testing and plans to roll out the Mango update to existing devices on Tuesday October 11. According to sources familiar with AT&T’s plans, HTC HD7S devices will begin to see update notifications on Tuesday. AT&T is still testing the Samsung Focus 1.4 devices so it’s not likely that these devices will see the Mango update on Tuesday. Here’s the internal memo sent to AT&T staff this week:

On October 11, 2011, Microsoft is scheduled to begin rolling out the new Mango 7720 update to AT&T customers with existing HTC HD7S Windows Phone 7 devices. The update supports the same enhancements that began rolling out to AT&T customers using HTC Surround, LG Quantum, and Samsung Focus devices in late September.

Important: To help ensure quality control, Microsoft is releasing the update in controlled batch quantities over several days. During the initial rollout period, the update is only available to customers once they receive a notification that an update is available on their device. The update will be available to all AT&T customers with existing HTC HD7S, HTC Surround, LG Quantum, and Samsung Focus devices after the controlled rollout has completed.

The new Microsoft Mango update for the current WP7 devices requires a two-phase installation. The update installation process includes:

• Installing Zune software to a computer.

• Connecting the device to the computer and installing the Mango update that adds new device functionality.

Note: If the Zune software has been previously installed, it does not have to be reinstalled prior to connecting the device to the computer to receive the Mango update.”

Microsoft recently revealed last week that Windows Phone 7.5 is now available for 50% of devices. The software giant started distributing its Windows Phone 7.5 “Mango” update to existing devices less than two weeks ago. “Within the first hours of making it available, we saw successful updates in every country, on every carrier, and for nearly every Windows Phone model,” revealed Microsoft’s Eric Hautala in a blog post last week.

Microsoft’s Windows Phone 7.5 “Mango” update contains over 500 new features. The software maker has included an updated HTML5 Internet Explorer 9 browser, multitasking and built in Facebook and Windows Live Messenger support.