Not everybody has made the jump to current gen yet, and Activision seems to understand that perfectly well. They're taking it so well in fact, that they're giving you a free upgrade to the current gen copy (PS4/XBOX ONE) of the title if you choose to download it on PS3/360!
The deal is pretty simple: If you purchase Advanced Warfare through Xbox Live or the PlayStation Store for a last-gen system, you can upgrade to the new-gen version for free. As long as you purchase the digital PS3 or 360 version by March 31, 2015, you're eligible. It doesn't work with the physical versions of the game, however.
For instance, let's say you don't have a Xbox One yet, but plan on getting one in the near future (or receiving one as a gift). In that case, you can download Advanced Warfare for the 360 and start playing, then upgrade later. You have to stick with either Microsoft or Sony, though; you can't buy the PS3 version and then upgrade to Xbox One.
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Keep in mind, that Sony's Crossplay offer does apply here. If you decide to download the game on PS4, it will be available on ps3, and vice-versa. However, if you Buy the Xbox One version first, you will not have access to it on your 360. It will only go one way, by buying and downloading the game on your 360, and then upgrading to the Xbox One version.
This is a fantastic offer to entice people that are still waiting to buy a current gen console to still pick up the game on their ps3/360 and upgrade later, especially with the Holidays being so close. No word yet on if this will be a timed exclusive deal, much like last year's Call of Duty: Ghosts offer.