Rather than swapping the pedal arms, I would suggest swapping the pedal axles, so you can keep the arms, but swap out the one pedal that is 1/2 to a 9/16 axle. I've done this before for swapping BMX pedals to work on mountain bikes. If you can't find any 9/16 pedals you want to extract the axle out of, or can't get anything on the internet, I've got a couple axles kicking around, prextracted. But do measure the length of the axle before you buy