What does the upgrade button in the advanced warfare building do?

My question is not about the random perks that soldiers get when they are promoted while you have an AWC built (like here).

My question is about the button appearing at the top of the AWC to change the soldier abilities. What does the feature at the top do?

The feature is to retrain a class's perks. Meaning, if you have a Ranger with your skills already set in place, say you have a Blademaster loadout, and you'd like to change up the perks, after 10 days in the AWC your soldier will keep their rank, and will be able to reselect the perks in each category they've unlocked thus far.