The area of the bottom of the 8" pan is (near as makes no difference) 50 sq inches. The area of the 9" pan is 62.5 sq inches - 25% more.
You can use the larger pan but since the layer of batter will be thinner it will bake faster. I would lower the oven temp a bit - maybe 25 degrees or so and watch the cake carefully for doneness. You know, the usual springy top, just coming away from the sides of the pan, clean toothpick, etc.
Jock |