User blog comment:Xilinoc/Ch. 540 - The Sword Five/@comment-8295743-20130605172815

Okay, the whole thing about Zangetsu is very, very simple. Ichigo has been a hybrid Shinigami for most of his career. His Hollow spirit fused with this Shinigami powers(aka Zangetsu) when he achieved Bankai. He lost those powers when he went all Super Saiyan 4 on Aizen. His Asauchi was in the form of Hollow!Ichigo because Zangetsu himself had NOT been separated from the Hollow. I think it still existed, but was powerless. Or at least unable to use his power. And now Zangetsu is being imparted upon a new sword, and not through the normal method. But now Ichigo's Quincy powers are slowly awakening. Oetsu is fusing the Quincy part of his bloodline with the Shinigami part, so of course Zangetsu's form is changing. And it makes sense that the Quincy-fused Zangetsu would look exactly like Yhwach, since technically Ichigo is a descendant.

Makes sense, no?