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Dave Meltzer was writing about it in 1999. The plan was to turn Austin heel at Wrestlemania 2000 and he'd feud with the Rock as the new company babyface. However, Austin's neck was breaking down and he needed to take 11 months off instead.
So when he returned in October 2000, they simply decided to go back to the plan. However, this time the Rock was already in ascendant and he took time off after Wrestlemania X7 to do Scorpion King and didn't return until July 2001. If you're watching Austin's heel run in 2001 and you think "god this sucks" its because two years prior they wrote it all out with the Rock as the babyface in mind, but he wasn't there.
Seriously just look up WON dirtsheets from 1999 because Meltzer's sources told him all about it in advance.