I agree, changing just the smallest chainring is easier. 2.1 tires will do well on the Katy Trail - I don't know what tread you have, but it doesn't have to be very aggressive. Larger air volume, somewhat lower pressure works well on the Katy's surface. I also find a 36 chainring works well for me on my touring bike in flat land (which is what you have mostly between Indiana and Missouri), but that's purely subjective.