I have 3 Window phones that are on the insider program, I have one (Nokia 520) that has the 10586.318 build. The other 2 have the Redstone build. Windows Central mentioned that the 512 MB ram phones won't be able to handle Windows 10, yet my Nokia 520 has it. I have a 32 GB SD card in the phone, don't know if that makes the difference.
Seems like older phones could get Windows 10, but not the software package that the newer phones receive. Then when the people with older phones buy newer phones, it would be an
easier transition to that new phone, software wise.