Microsoft has yet to begin the official over-the-air update of its older Lumia smartphones to Windows 10 Mobile. However, a recent support page for T-Mobile Germany indicates they expect the updates to bring for several of its Lumia phones to begin very soon.
The support page shows that the older phones scheduled for updates to Windows 10 Mobile on T-Mobile Germany are the Lumia 535, 635, 640, 640 XL, 830 and 930. However, the page does add that the update could take as long as two weeks to appear on Microsoft's servers. Also, the update will be staggered when it begins so as not to overload the download servers.
Keep in mind that schedules for software updates of any kind can, and most likely will, change. So while this list is an official one from a wireless carrier, it should be taken with a grain of salt.