Zeref is a colossal mass of contradictions thanks to his curse. He almost certainly wants Lumen Historie because it's Mavis,not because it's a weapon. However,he also wants it because it's a weapon.
Honestly,I wouldn't call this a retcon. Retcon means a retroactive continuity. There is a media,like a book,a videogame,a movie,a play,etc. When you retroactively change the meaning behind a scene,or even change the scene entirely,or add contradicting information to a caracter or place's background you have a retcon. It must not be foreshadowed,and it must create a contradiction.
Yuriy being Serena wouldn't be a retcon,it would be a plot twist or a revelation. You would be elucidating a caracter's backstory without contradicting previous information.