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(Pocket-lint) - Microsoft's annual Build conference is about to kick-off later today and with it should come an unveiling of Windows Phone 8.1, at last.

UPDATE: We've now had hands-in time with Windows Phone 8.1, you can find all the details here: Windows Phone 8.1 preview

The latest mobile operating system from Microsoft has been a long time coming with Windows Phone 8 released back on 29 October 2012. So what has Microsoft been working on all that time?

Windows Phone 8 has had three updates during its life, but WP 8.1 is a larger build, which should mean huge changes. Here's what we expect from the Windows Phone 8.1 "Blue" update.

What updates will Windows Phone 8.1 bring?

An SDK of the WP 8.1 OS leaked a while ago so plenty of changes have already been reported. But of course they're not official at this stage.

An Action Centre should be added that finally allows users to pull-down a menu from the top, much like Android and iOS offer. This will display notifications that can be adjusted to suit the user's needs. The quick access part of the pull-down will be customisable to display four tabs at a time which can be changed in the settings. Brightness, Wi-Fi, messaging and phone settings can all be toggled from this menu.

Swype input on Microsoft's stock keyboard is expected to be introduced as an alternative to typing. It will also be smart enough to suggest emoticons based on the sentence being written.

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Cortana voice assistant will be accessible from the search button. Much like Siri or Google Now, it should offer search and phone controls through voice commands.

Apps and games could be stored on SD cards freeing up memory on the handset. This will only work with one originating handset so they can't be shared, in case you were getting excited. Store apps will update automatically if the user turns that option on.

Google Calendar will get support allowing users to have multiple feeds in the native calendar app. This will also have week view added and will even pull in the weather.

The camera should get a new layout and add a burst camera mode to its feature list.

A newer podcast app will give more support to users. It will include options like variable playback speeds, favourites, audio and video support, Bing search to find and add podcasts, plus sharing support.

Other updates include smarter email syncing to save battery, improved Internet Explorer 11 to work with HTML5, password protection for Office and an ability to scan documents to import to Office.

The back button no longer closes but suspends apps, a double tap will unlock the phone, and Kid's corner offers a secure area.

Live tiles that can be marked as read by swiping over them should be introduced, and the ability to change the default messaging app will be added. There's also smarter Bluetooth 4.0 and better battery saving options in the Battery Power Sense app. Support will also be added for stereoscopic 3D, mouse and keyboard, NFC wallet, and Miracast.

Why it should be coming soon

According to WPCentral the Windows Phone 8.1 build has not only been completed but it's already in the hands of phone developers. Apparently the Windows Phone 8.1 software was sent out to OEMs on 26 March with a plan to release by 23 April.

Which phones will get WP 8.1 first?

It looks very likely that Nokia will be unveiling a new handset during Build. This should come with the new Windows Phone 8.1 update. The rumour mill suggests a Nokia Lumia 930 is on its way to upgrade the Lumia 925. Whether that will have upgraded hardware as well as the OS is still unclear.

Other handsets that could feature the WP 8.1 OS immediately include a Lumia 630 and Samsung ATIV SE.

How will this affect developers?

Microsoft will apparently allow anyone with a Developer Preview app and an App Studio account to download and run WP 8.1 on their device.

This will be primarily aimed at developers but of course anyone who's enthusiastic about the update and can't wait will be able to upgrade early. This is expected to roll out later in April suggesting older handsets won't get the over-the-air update until May at the earliest.

Writing by Luke Edwards. Originally published on 2 April 2014.