Wait, what? Upgrade? Or do we mean "to obtain"? Last I checked you can't further "upgrade" or augment the wkr drops, they are as they are or I'm missing something that no one else has mentioned before.
Christ! Why doesn't any page reference this apart from forums? This looks awesome.
They are awesome. I love my Occasionally Attacks Twice bow with +2% Critical Damage. And my OAT crossbow with +10 MAB is fun for spamming Trueflight and Wildfire.