In "A New Hope" we see Leia recording a message into R2-D2's memory before she sends him down to the surface. Vader's men inspect the computers on the Tantive but find nothing, suggesting that the plans were being physically carried.
So was R2-D2 always the carrier of those plans (e.g. from the point that they were received, onward) or did Leia put the plans into Artoo at the same time she recorded the holo-message?