Microsoft has announced through their Lumia Facebook page that the global rollout of Windows 10 Mobile will begin this December, bringing the company's completely revamped mobile operating system to older Windows Phones before the end of the year.
The statement from a Microsoft representative on Windows 10 Mobile upgrades is as follows:
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.
As Windows 10 Mobile won't be coming to older Windows Phones until December, there will be a short period of time where the new Lumia 950 and Lumia 950 XL are the only devices running the new OS. Microsoft has already announced that both their new flagships will be hitting the market in November, and of course they'll come pre-loaded with Windows 10 Mobile.
It's not completely clear which Windows Phones will get an upgrade to Windows 10, but Microsoft are hoping to upgrade "the majority of Lumia smartphones" to the new OS in the near future. Unfortunately Windows 10 Mobile does require 8 GB of internal storage, so there are already some low-end handsets, such as the Lumia 530, that won't be eligible for an upgrade.
Not every Windows Phone will support every Windows 10 Mobile feature, either. Windows Hello and Continuum will require new hardware in the majority of cases, while naturally some features will only be available on flagship devices. However judging by early builds, Windows 10 Mobile runs pretty well on a wide variety of hardware, and there will be enough new features to make it worth upgrading to on any compatible device.