Microsoft's latest mobile OS will be introduced to the public by way of the new Windows 10 smartphones. They are the Microsoft Lumia 950, Lumia 950 XL and Lumia 550. The devices are expected to release sometime later this month or early next month.

In anticipation for the Microsoft flagships, many are also wondering when the Windows 10 Mobile would be rolled out for older Lumia-branded handsets that are currently running on Windows Phone 8.1.

There were several reports in the past claiming that the Windows 10 update for older Lumia models would commence in December 2015. The December 2015 rollout date has been confirmed by Microsoft itself on the official Lumia Facebook page.

"Windows 10 is available on PCs and tablets now, and while Lumia 950 and Lumia 950 XL ship with Windows 10 for phones, broad availability of Windows 10 on other devices, including mobile phones, will begin rolling out in December," commented Microsoft in response to a fan query.

The confirmation was music to the ears of Lumia owners. However, "December" isn't really much of a precise answer. Fortunately, actual update dates have now been reported.

According to WPWorld, with translation from WMPowerUser, mobile carrier Orange Poland has revealed two different waves of Windows 10 Mobile updates coming to their lineup of Lumias.

The first one is scheduled on Dec. 14. The handsets that will be treated with Microsoft's latest mobile OS are the Lumia 630, 635, 640, 735 and 830. Unfortunately, the second wave of updates will come in early 2016, Feb. 1 to be exact. This wave includes the Lumia 520, 532, 625, 720, 820, 920, 925, 930, 1020 and 1320.

Interestingly enough, there was no mention of the Lumia 1520 receiving the update. This is probably because it was sold in Poland as an unlocked model, so the carrier had no participation with its update process.

Bear in mind that if the dates turn out to be true, they only apply to the Lumias being offered by Orange in Poland. Hopefully, the timeframe is also similar to that of other carriers around the world.

Microsoft previously said that it's the company's ambition that majority of Lumia smartphones will receive the update. It should be noted that 8GB of native storage is required to download the firmware, meaning owners of devices such as the Lumia 530 have some serious hard drive freeing up to do.

Only Lumias running on Lumia Denim will be able to upgrade to Windows 10 Mobile, and "not every phone will support all possible Windows 10 features," as per a Microsoft support page.