So... I don't know how many of you saw the movie, but a major point of vampire physiology canon got changed.
Stephenie clearly stated here
that vampire hair does NOT CHANGE during transformation. Not at all. But in the movies, her hair changes as she transforms.
Interesting, isn't it?
I think the others are probably right in this respect. I don't think her hair changed so much as it repaired. Let's face it, overall after the pregnancy and delivery, she looks ragged, worn, wasted away. Her hair is brittle and dull, almost gray. Her lips are pale and blend in with her face which also has a gray tone. She is horrid. But the venom is supposed to restore the health; with the exception of the paleness. So everything that she went through during the pregnancy would be corrected. Including her expanded belly. I'm sure that Edward was in similar since he had been ill prior to his transformation. The others were changed rather quickly after the incident that should have resulted in their death. So their transformation may not have been as visually enhanced. Yes, their injuries were repaired but overall, their flesh didn't have to fill in again. I think Bella probably went further towards her own natural (or unnatural) death than any of the others so that was why the change had to be drastic. Her heart stopped so she actually did die... None of the rest of them had gone through such a lengthy downward spiral to their death as she had. She changed physically as much as humanly possible before she transformed. The venom had a lot to correct.
Keep in mind, what we were seeing was taking place over several days. It almost certainly didn't happen as rapidly as we saw in the movie. I believe that was just for the visual affect and comparison. I remember a guy sitting near me at the theater saying, "She didn't have make up on a second ago."
I'm thinking, "Idiot, you aren't supposed to notice that..." Like the dieters with the before and after shot. They almost always have a tan in their after shots...
I've always noticed that Edward has a mole on the nape of his neck, below his hair line. But that's never bothered me as much as Alice cutting her hair off. They are physically incapable of change, so there would really be no way she would cut her hair so short. It would never grow back out. But then again, Edward shortened his hair as well...