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This post on there support site confirms it:
"GTA: San Andreas on the Xbox Marketplace will be available again for purchase beginning October 26th, 2014, through Games on Demand. This version features 720p resolution, enhanced draw distance, and Achievement support."
If you brought the game previously, Rockstar states: "The Games on Demand version of San Andreas is not compatible with the Xbox Originals version. Save files will not transfer from Xbox Originals. Owners of the Xbox Originals version will still have access to the old version on their consoles and through their download history, with the option to purchase the new version to experience the added features on a new save file."
Also GTA V comes to XBOX One and PS4 this November 18th, PC version in January 2015!
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