Memory Alpha's article on "Datalore" gives the dates of many states of its creation:
The first story outline was 22 july 1987, and the episode was filmed 28 October to 6 November 1987. And it is possible that some of the dialog was later changed and dubbed over what had been previously said. But I doubt that all the statements establishing that Data was built by the human Dr. Soong were dubbed over statements that Data was created by aliens.
So it seems 99.99 percent certain that the creation of data by Dr. Soong was decided on before filming started.
But Data was not created by Dr. Soong in the first outline on July 22, 1987, since the question notes that the final draft of the Series bible is dated September 8, 1987 (the 21st anniversary of the first TOS episode) and describes Data as having been created by aliens.
Thus the backstory should have been changed sometime between 8 September and 28 October 1987. Data's creation by Dr. Soong might have been decided on between the final draft on 19 October and the revised final draft on 26 October. But I think that such an important plot element was probably decided before the final draft was written.
I expect that Memory Alpha omits a number of story outlines and scrift drafts before the final draft, and one of them, or memos relating to one of them, might include the decision to make Data created by Dr. Soong instead of aliens.