It looks like the upgrade to Windows 10 has caused a lot of confusion to everyone interested.
That is why Microsoft has published a new app, which will tell you if you phone is able to get one. The app answers questions about Windows 10 and will give you tips about how to prepare your device for the new firmware (if your phone can get one at all).
From what we know, Lumia needs 8 GB of storage for the new Windows.