In my opinion trans fluid change is easier/cheaper than a regular oil change on the Si...i have a CAI so it makes it easier to just fill from the top...20k-30k miles
According the the Civic Manual, when the maintenance minder says "A3" or "B3" and every 25,000 miles in mountainous, towing or slow speed conditions. I made a video how to change the fluid you can watch here: View: https://youtu.be/AGjgfQWRSN0