It's still a cool idea, nontheless, but wouldn't that add needless turnaround time to production?
Not necessarily, or at least not by much. They unified the programming for the 360 with DirectX 10, so it makes sense that they'll keep the code the same for the next version of the xbox, with the exception of adding a few new commands to speed up certain functions (it should include whatever commands get added to DX11, but the point is DX 10 will still be able to run on DX 11 systems). This means there is no extra time added for making the code compatible with the new xbox.
Since the code is the same, then all they would have to do is change a few variables (such as depth of field, number of light sources to render, etc.), models, and textures to be higher resolution. When developing models and textures for games, they often create them at a much higher resolution then the game system can handle and downgrade them to what they need, so this means including the higher resolution models and textures should be fairly simple for most games. It's similar to how PC games are currently made, they'll just include a higher graphics setting for the new system.