Why change it? Because it can easily get even better and it's not so much the bowling that has been so successful in the past 2 tests, it's been the fast bowling. They've done all the damage, because the pitches have suited them and it could easily be a similar situation for this next game.
Put it this way, who would India rather be facing, if it's the sort of grassy, seaming and swinging pitch that the SCG has apparently been in recent times (and was for the same test last summer, when spin got just 3 of the 30 wickets, despite Swann playing)? Lyon, who they have very little trouble with, if at all, or one of the best fast bowling options in the world, who could easily rip through their top order like the other three did in the last match?
If it is another grassy pitch, they will be hoping and praying we pick Lyon, I expect.