According to the Twitter’s tweet of Joe Belfiore, Microsoft is now working on the Windows 10 to work with any phone with just 512MB of RAM, including Lumia 520.
And the Microsoft’s ambition is to make Windows 10 for Phone available for those devices, but there were some differences in features.
Hey all: we ARE working on Win10 for 512MB, including 520. Our ambition is to make #Win10 available for these devices but features may vary
— joebelfiore (@joebelfiore) February 8, 2015
As he said, the Windows 10 Developer Preview for Phone will be available in this month, but the team had to complete the test before release it to others.
So at least, Microsoft will give us more than one version of the Windows 10 for Phone, one for high performance smartphones and the other for low & budget phones with less hardware.