Supported Phones: Lumia 630 family, Lumia 730 and Lumia 830

This is bit shocking but…

Supported phones for Windows 10 Technical Preview:

We’re working to get more Windows Phone models supported worldwide. Download the Windows Insider app, and when your phone is supported you’ll be able to sign in with the same Microsoft account you used to join the Windows Insider Program. Windows 10 Technical Preview currently works with the following phones—as long as they have at least 8 GB of storage and Windows Phone 8.1 was already installed on the phone at the time of purchase: