User blog comment:SociallyAwkwardEpic/GTA: San Andreas getting a re-release?/@comment-25753101-20141022190916/@comment-24438637-20141104192721

That is because this will be a new game that releases to the Marketplace on October 26th. It will be a separate GTA: SA, so you'll have to beat it again to gain the achivements. This link will explain more, Rockstar's official link:

https://support.rockstargames.com/hc/en-us/articles/204078933-Games-on-Demand-GTA-San-Andreas-Replacing-Xbox-Originals-Version