The Torah calls last day of Sukkos Shmini-Atzeres (or techically a separate holiday (PZ"R KS"B)) but at some point in time it became Simchat-Torah (in Israel) or the 2nd day in Chutz L'aretz.
When did this happen ?
I think the question should be : when Shmini Atzeres became ALSO simchat Torah.It is Ezra tha decided to read the all Torah not anymore in 3 years but in one year.He decided to make it a simh'a and choose Shmini Atzeres. Why ? I have yo look bn