It's not that simple. Everyone. EVERYONE. Including the current Pope is BOUND to assent to Scripture, Tradition and the Magisterium that came before. If there are parts of Magisterium that are reformable, then a current Pope can reform them. But for the current Pope to reform this matter, he would have to show why Scripture, Tradition, and the Popes before him were all wrong, and the theological reasons behind the rejection of Zionism are, unfortunately for him, irreformable. Thus, he cannot change them if he wanted to. Thus, the Church teaching against Zionism will always stand.