Microsoft has started sharing its Windows Phone 8.1 Software Development Kit (SDK) with developers, and the inevitable leaks have begun. Over on Reddit, one developer has been documenting all of the updates discovered so far. The first major change appears to be the initial signs of a Windows Phone and Windows RT merger. While iOS and Android both allow developers to build apps across phones and tablets, Microsoft doesn't yet, but it has been hinting that it would follow a similar path. Windows Phones 8.1 includes support for "Universal Apps" that let developers build Windows Store and Windows Phone Store apps from the same shared HTML and JavaScript code. Effectively, this means Windows Phone 8.1 and Windows 8.1 can run near-identical applications.

It doesn't appear to be a full merge of Windows Phone and Windows RT just yet, but the early signs suggest Microsoft will move the two operating systems a lot closer in the future to the point where apps work regardless of the platform or device. Microsoft is also switching to the .appx packaging for Windows Phone 8.1, the same format that the company uses to distribute Windows 8 applications. When you combine all these changes together, it's clear the merging of Windows Phone and Windows RT is an area of focus for Microsoft with this latest version.

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Microsoft also appears to be readying changes to allow Windows Phone 8.1 users to switch out their default messaging application. Similar to customization capabilities in Android, a screenshot shows a setting to "receive texts using" different apps. Microsoft previously made a similar change in Windows Phone 8 to allow developers to create replacement camera apps that could be set as the default. Other improvements include a new "Battery Power Sense" feature to finally monitor and track battery life, VPN support (a big deal for corporate users), and the obvious rebranding of SkyDrive to OneDrive throughout the OS. A new "Storage Sense" option also shows that Windows Phone 8.1 will support installing apps to SD card.

The Reddit poster also reveals that, as previously reported, the Music and Video apps in Windows Phone 8.1 have been separated out, and the combined Music+Video app has been removed. There’s also reportedly a new podcast app, but it’s not clear if this will finally support international markets.

A new built-in camera with burst mode

Microsoft is making some changes to multitasking, too: developers are reporting that the back button no longer terminates the application, but suspends it in the background like in Windows 8. It's not clear how much closer this is to iOS or Android-style multitasking, but it's clear Microsoft is seeking to rectify some of its existing issues here. Other multitasking improvements include a quick swipe down to close apps, so you no longer have to tap on the close button. Microsoft is also tweaking its built-in camera app — the new style looks similar to Nokia's Camera application, with a top app bar providing access to Lenses, flash, and front / rear camera selection. Meanwhile, the app's bottom nav bar switches between a taking a photo, capturing video, and toggling a new burst mode.

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Facebook account setting missing in Windows Phone 8.1

Some of the bigger settings changes in Windows Phone 8.1 include the addition of an iCloud account option, and the removal of the built-in Facebook integration along with support for Facebook in the Photos app. However, Twitter is still present — so it's not clear if this is a bug in the SDK or a sign that Microsoft is taking a step away from Facebook like it has with Windows 8.1. It's possible that a separate future Facebook app could integrate itself into the hubs once installed.

Internet Explorer has also been updated in Windows Phone 8.1 to version 11. Microsoft has been sparring with Google over a YouTube app for Windows Phone, and the 8.1 update appears to change the behavior of YouTube videos thanks to the new browser update. Developers report that embedded YouTube videos simply play on the page now, instead of the separate fullscreen video controls that Microsoft has used previously.

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On-screen Windows Phone 8.1 buttons that swipe away

Nokia is planning at least one Windows Phone 8.1 device without physical navigation buttons, and the support for on-screen buttons is present in the new SDK. Previous leaks have demonstrated the capability, but today's screenshots show that there's also an arrow at the side of the buttons to allow users to slide them away so you have more screen real estate for apps. On-screen buttons can then be reactivated with swipe up from the bottom of the display.

While Microsoft is also building a voice-activated "Cortana" personal assistant into Windows Phone 8.1, it appears that the company is still keeping that secret. Developers report that the search button and any Bing-related functionality in the new SDK results in errors, a sign that Microsoft has likely blocked access to this functionality. Microsoft's new notification center, referred to as Action Center, is also not present in the SDK.

Microsoft is planning to release the final version of Windows Phone 8.1 as a preview release during the company's developer-focused Build 2014 conference in April. Today's leaks don't detail all of the new Windows Phone 8.1 features, but we'll update you accordingly if any additional changes are discovered.

Update: Twitter user @AngelWZR has started posting more screenshots, including the separate volume controls in Windows Phone 8.1. We've updated the gallery below with additional screenshots.