You might find that the thinner line works better under windy conditions as it will be slightly less air resistant. You will still be able to load the rod but it will take more line out of the end to have the weight of line the manufacturer intended to load the rod. To be truthful you might not notice much difference with most graphite rods. It may be a little harder to feel the rod loading which seems to be one of the main reasons people give for going the other way and overlining a rod. Try borrowing a lighter line and trying it on your rod to see how you feel about it.