Can I use smush pro on my current site?

I am learning more and more about WP sites...recently I ran two test one using Yellow Lab Tools and the other PageSpeedInsights....my scores came back AWFUL. One of the reasons is due to image compression issues. If I run Smush Pro on a live site, what impact will it have?

Do I have to update every image I have on the site or does it update it automaticallY?