Given that handedness is not a minor thing and is tied to eye dominance and other things, I always figured that they designed them so that while not doable in combat the technicians could change out the lower arms and move them about. So when a trooper was assigned to a specific suit then the techs would do the various adjustments you'd need to make to finetune the inside of the boots, joint position and flexion, etc. And while they were at it they could swap the lower arm assemblies between arms to account for whether a trooper was right or left handed.