Nobody knows yet. My guess is that hardware requirements won't be an issue, however.
The limiting factor here is probably going to be carriers and whether or not manufacturers are going to push out new firmware. If you're on a Lumia running Windows Phone 8.X then I wouldn't worry about the latter problem, though it could be a while before you get the update if you're not on the developer preview program.
Nothing fundamental is changing about the OS here. Under the covers it's going to be virtually identical, or at least close enough to not radically alter the hardware requirements, especially since the low end is still the volume market for Microsoft and Windows Phone in particular.