Originally Posted by
rm -rf
It's easy to switch cassettes, you need a cassette tool and a chain whip. I swapped out my 12-29 for a 12-25 for my quite flat ride tomorrow.
This. Cassette, $30, two tool, $30, and they will be useful for as long as you ride a bike.
Since you have the 11-28, no need to buy the 12-29. Just pick up the 12-25.
In fact, you could just ride what yo have and see how often you use the bottom two gears. A lot? Ride on. Almost never? get a second cassette and much better gearing for the flats.
Related fact: I ride a 50-34 with 11-28 on almost entirely flat terrain and it is fine. I might do the 12-25 myself, but likely unless you hit a lot of wind or want to ride with fast groups you are all set with what you have.