Ben switching to Leia makes more sense, though, since she begins to play a regular role immediately around the time Obi-Wan dies. Biggs is only onscreen for a handful of minutes, while Lando doesn't appear until very late in the second movie, with absolutely huge gaps where neither appear -- plus, both are pretty minor roles overall, and there are already more major obviously-PC roles than there are players.
(It would be amusing of Sally played Luke, since this would fit the brother-and-sister thing. But it seems unlikely when there's so many other characters she could play.)
I initially thought so as well, but since Leia, R2 and C3PO are important characters straight from the beginning, her being a PC from the start is not improbable.