From what we think, it seems Microsoft may go for Quad-core + 512 MB RAM or Dual-Core + 1 GB RAM as the minimum requirement for support for the first Windows 10 for Phones preview builds and with Builds progressing it will include most of the devices ultimately. So, while Lumia 630 / Lumia 635 should make the cut, Lumia 520 users may have to wait. Though let us wait for the list to be out.
@NicholasHayman I wish I could answer. We’re finalizing the list of supported devices for the first preview. Will share soon.
— Gabriel Aul (@GabeAul) February 5, 2015
You can read all about the changes and new features that Windows 10 for Phones brings with itself at our dedicated Windows 10 page.