Microsoft has not officially stated when Windows 10 Mobile will begin to roll out as an update to older Windows Phone devices. However, a slide from the France-based smartphone company SFR claims that it will begin rolling out to several Lumia smartphones sometime in October or November.
Specifically, the slide shows that Windows 10 Mobile will be provided to the Lumia 435, 520, 530, 625, 635, 830, 920, 925, 930, 1020 and 1520 models. Previous rumors claim that Microsoft plans to release Windows 10 Mobile to smartphones OEM sometime in late September or October.
It's important to point out that schedules can, and likely will, change for Windows 10 Mobile's release. Even if the update begins to roll out in the fall time frame, it could be several months before the majority of Windows Phones will be able to access Windows 10 Mobile.