If you have brake levers which work, why replace them? You can install some trigger shifters like these: https://www.amazon.com/Shimano-SL-M3.../dp/B01FJ4XQVG, assuming of course that you have Shimano derailleurs. I have installed those on two of my bikes and they work great. I prefer separate brake and shift levers since I can set the angles and positions separately.