Why was speaking to Greeks a change for these Jewish believers?
Speaking to Greeks was a change because these Jewish believers had previously preached to Jews only. A Grecian was a Jew not native to Palestine, so that was familiar. A Greek, however, was a Gentile. Approaching Gentiles was a new venue and modus operandi, as God began moving the Gospel toward the Gentile world.
Les explains that the word in the passage should be understood as Greeks, not Grecians. A Grecian was a Jew who was a citizen of another nation outside Palestine, so it was not unusual for Jewish believers to approach Grecians with the Gospel. A Greek, by contrast, is a Gentile. Therefore, speaking to Greeks was a complete change in venue and modus operandi, because previously they had not been talking to Greeks. This shift fit God's plan: He started moving toward the Gentile world when He saved Saul, then sped it up when He sent Peter to Cornelius. Now Saul had returned from three years of learning and the revelation of the mysteries, and the Gospel was to go out to the Gentiles. These Jews who had preached to Jews only began speaking to Greeks and preaching the Lord Jesus, and the hand of the Lord was with them, so a great number believed and turned to the Lord.