Sentence:  His addle-brained knight-errant , self appointed to the ridiculous position in an age when armor had already been relegated to museums and the chivalrous code of knight-errantry had <WSD>become</WSD> a joke , is , as Cervantes no doubt intended , a gaunt but gracious symbol of good , moving soberly and sincerely in a world of cynics , hypocrites and rogues . 
Meaning of the word marked with <WSD> tags:  undergo a change or development