How can you add HDR to titles that never had it? It seems like it would be mapping a smaller set of SDR values to HDR values, but how do you do that? If, for example, you now have 256 different shades of HDR grey how do you know which value to map the 64 shades of SDR grey to? I'd think SDR games would only use an SDR palette for their artwork (e.g. texture maps).
Compatibility with Xbox360 games would be interesting considering its built on the PowerPC architecture and the graphics processing relied on its eDRAM. Maybe in that case they would just port the PC version across?
It will be interesting to see how much power the Xbox Series X really has for native games.