Why did Saul's name change to Paul?
The name Saul is Hebrew, and the Greek version of that name is Paul. At a point in the book of Acts, Luke begins addressing Saul by his Greek name, Paul:
From this point forward in Acts, Luke uses only Paul’s Greek name, reflecting his assignment to preach the Gospel to the Gentiles. So Paul’s name was never changed, but Luke chooses to use his Greek name rather than his Hebrew name to reflect Paul’s mission to reach Gentiles with the Gospel.