Getting out of going would be easy. Getting out of being a groomsman is gonna be tough. You need a major excuse, especially after having confirmed. Can always fake sick the day off I guess, but it'll have to be something major and you'll have to make it believable (lot of details), and involved lies like that are inherently risky and have a habit of biting most people in the ass later on, especially with something big like a wedding.
Personally I'd just go. It's just a few hours of your time and you've already confirmed so...