Microsoft finally begins Windows 10 Mobile roll-out

Microsoft has finally begun rolling out Windows 10 Mobile to older Windows Phones, months after the operating system first appeared on the company's flagship devices, the Lumia 950 and Lumia 950 XL.
Windows 10 Mobile is being delivered to a collection of older devices via an over-the-air update. Users with an eligible handset running Windows Phone 8.1 can download the Windows 10 Upgrade Advisor app from the Windows Store, which will check if an update is available, before heading to the Phone Update section of the settings screen to initiate the download and installation process.
Despite Microsoft initially promising that all Windows Phone 8.1 devices would be upgraded to Windows 10 Mobile, only some are eligible to be updated today. The list of eligible handsets mostly includes recent Windows Phones, including the Lumia 1520, 930, 640, 730, 830, 540 and 430, while older devices like the Lumia x20 series (eg. the Lumia 920) aren't getting the update just yet.
Microsoft's reasoning to leaving out older handsets is as follows:

"As Windows 10 delivers significant new innovations, many older devices are not able to successfully upgrade without an impact on the customer experience. Our goal is to only offer the Windows 10 upgrade to devices that we are confident can continue to deliver a good customer experience."

At least Windows 10 Mobile is actually rolling out to some users now after a long wait, and those with newer Windows Phones will be able to enjoy the new features the operating system brings.