The big day has finally come, as rumored mere hours ago. It's not a development version anymore, it's not a beta, it's not a test rollout, it's not meant for Windows Insiders only, it's the real deal. Microsoft has just announced the start of the rollout of Windows 10 Mobile to older smartphones that are currently on Windows Phone 8.1. Needless to say, this has been many months in the making, and from this point on updates should be hitting phones left and right.
The update is headed to these smartphones, though each one may get it on a different date (and where Microsoft/Nokia isn't the manufacturer, things are up to that particular company when it comes to timing): Lumia 1520, 930, 640, 640XL, 730, 735, 830, 532, 535, 540, 635 1GB, 636 1GB, 638 1GB, 430, 435, BLU Win HD w510u, BLU Win HD LTE x150q, MCJ Madosma Q501.
Unlike what usually happens with over-the-air releases, there is something you need to do to ensure you get Windows 10 on your device (if it's one of the above, of course). That is to download Microsoft's UpdateAdvisor app from the Windows store. This will confirm that the update is on its way. Once you open it and it does that, make sure you check the "Enable Windows 10 upgrade" option and tap on Next. Then when it says it's ready to upgrade, just tap Done.
Only after you've gone through this procedure does it make sense for you to go to Settings > Phone Update and check for updates. If you've completed all the aforementioned steps, then you'll receive a notification once the OTA has reached your particular handset. If you have a Lumia the new software will probably arrive sooner than if you carry another brand, but nothing's 100% certain in this regard. One thing is clear, though - Windows 10 Mobile is finally on its way to a smartphone near you.