Who's to say that event significantly changed the story? Let's say R5 goes to Luke and R2 hangs back with the Jawas. R2 is resourceful, so he would figure out a way to escape and find Obi-Wan on his own. He must have some record of Ben's location from Bail Organa, since he heads off in just the right direction from Luke's farm.
So, he finds Obi-Wan, plays the message, and Obi-Wan decides to help. He goes to the farm to pick up Luke, either before the Imps arrive or maybe Luke is off at Toshi Station picking up those power converters. Either way, ben has been waiting for the opportunity to confront Luke, so he convinces Luke to come with him and the rest of the story unfolds pretty much the same way.