Yes, it is obvious that some variation of that conversation took place pre-s8 (I mean from a logical POV, I doubt the writers gave it even a moment's thought), but the problem is that we *know* that Joss decided to make Dawn a giant because it was funny, not from any sort wish to explore Dawn's character. He *could* have used the transformation to do something, or show us something, or whatever, but the only thing that's happened so far is the setting up of Xander/Dawn. Maybe.

Basically I quite simply don't care. I'm more worried about whether Spider survives AtF than about Dawn's fate. Spider has had character development, has grown and changed and developed, concepts entirely alien to comic!Dawn I'm afraid.