Lich Bane should do what Runeglaive used to do and change the autoattack's damage to magical.

And make this effect an on-attack effect, and not on-hit (just the magic damage shift, not the bonus damage on-hit)

So Lich Bane can be better at taking down turrets with AAs and stuff and scale better with magic penetration (since that small part of physical damage from the autoattack doesn't benefit from magic penetratiom), but it wouldn't break AP Ezreal as his Q doesn't apply on-attack effects (like Runaan's Hurricane for example)