Remember how Microsoft said that "all Lumia Windows Phone 8 devices" would be updated to Windows 10? Now, after the Redmond giant showcased most of the new features that Windows 10 is bringing, the company isn't so sure anymore that all models would get the update.
According to a post from the official Lumia Conversations blog, which presents an interview with Chris Weber, Corporate Vice President of Sales for Microsoft Mobile Devices, the "goal is for the majority of the Lumia phones running Windows Phone 8 and 8.1 to join the Windows  ecosystem." That means some devices could be left out. In fact, this quote - "not every phone will upgrade or support all possible Windows 10 features" - is pretty self-explanatory.
While Microsoft mentions some of the recent Lumias that will certainly receive the Windows 10 update (like the Nokia Lumia 930, Nokia Lumia 735, or Microsoft Lumia 435), things aren't that clear when it comes to devices released a couple of years ago, like the Lumia 620 or Lumia 520. Hopefully, we'll soon have an exhaustive list of WP handsets that are Windows 10-ready.
Despite the fact that Microsoft says "there’s never been a better time than now to buy a Lumia", it's probably better to wait until newer models are launched - including a much-anticipated flagship that will run Windows 10 out of the box and is expected to arrive as soon as late summer.
source: Lumia Conversation