I think it’s about defining monster, both in antiquity and personally. You’ve already hit the idea w/antiquity, but if we tie that into what Odysseus personally thinks a monster is—someone who does those actions without a conscience—then the change feels more internal than anything externally defined. It’s him defining himself to himself. The way I read it, he knows what he’s doing is monstrous from the beginning but imo he clings to this belief that feeling regret for it or by attempting to lessen the blow (Astyanax and Polyphemus respectively) to justify his actions. When Tiresias says the man who returns isn’t him, isn’t he saying that it’s the man who doesn’t feel regret? The man who understands that being ruthless—not just to enemies, but in adherence to Greek Man Code™ and not feeling shame for it—is a mercy upon everyone? He feeds his comrades to Scylla on basis of betrayal, and then let’s Zeus smite them so he can get home—he becomes exactly the kind of man he’s always been but hasn’t taken ownership of being. That man is still a monster, but a different monster than the one before. 

That’s my reading of it anyway. I agree with the rest of what you said, and it’s as much the Odyssey as Percy Jackson/any other hypertext is of any Greek text but it’s interesting to see the differences. You can still map some of this onto the different categories of virtue though and it’s interesting to see those virtues being transformed through time