In A New Hope, Princess Leia sends R2-D2 and C3PO to Obi-Wan who is supposed to take R2 to Alderaan where Leia's father is supposed to decode the Death Star blueprints out of R2's memory.
Then Alderaan gets blown up, and Obi-Wan and Luke do not know about this until they actually reach the remaining pieces of Alderaan floating around in space.
I do not quite understand what happened after this .
R2 was supposed to go to Alderaan but it blew up, but in the movie the characters keep talking about taking the Death Star plans to its final destination.
At what point in the movie did R2's destination change?