i did not have a problem with the way she changed.
i was pretty much in tears, my heart racing from when bella is having Renesmee, to when Edward saves her with his venom. If he had just bitten her, i probably would not be freaking out as much. When I read it, i refused to look through the book. I did not look to see how many pages it was, or when i saw that it said Book 1: Bella, i did not want to see if there were other books with different points of view because i did not want to risk seeing anything that would ruin it for me, so i had no clue that bella would live after Renesmee, so when Edward was trying to save her, i had no idea whether or not he would succeed. Also, instead of Edward "killing" Bella by turning her into vampire, he was saving her. This way, Edward would never feel bad about changing her and it would never be something to feel guilty about.
Another thing i read in this thread was people speculating how Bella had an "easier" time handling the pain. She definately did not. If you read it, she is saying how unbearable the pain is and that she wished she had never been born and that she could not see how anything was worth that pain. So, it was definately not easier for her than it was for any of the other Cullens. The only difference, is that she had kept still because even if she twitched ever so slightly, she would lose all control.