LOS ANGELES—While Microsoft's pre-E3 press conference focused largely on newer video games, the event also filled in a pretty major gap for hardware-upgrading holdouts: backward compatibility. Starting later this year, the company's newest console, the Xbox One, will support a limited number of older Xbox 360 games—and Xbox One preview program users will get a shot even sooner than that. Gamers will have two ways of playing old games that are part of the backward-compatible initiative. If users already purchased the games digitally through Xbox Live, they can simply log in and re-download the game on Xbox One without paying any additional cost. If they own the game as a disc, they'll have to download the game to their Xbox One hard drive, and the system will then check for the disc before launching the game. Technical details on how this works are still unknown. The hardware of the Xbox 360 is very different from the hardware of the Xbox One, and pure emulation of th...