Warning: spoiler. Read only if you've watched the movie.
When Andy decided to undergo a make-over, she quickly realized that changing her clothes was only the start of her initiation into the world of fashion. The fashion industry demanded more and more time from her and gave her less and less time for her friends and her boyfriend. She even missed his birthday party. At one point, she was also made to choose between her friendship with her officemate and her job and she chose her job. In the end, she was made to realize that all those choices that she was making, which were in no way inconsequential, was changing her trajectory and making her more and more like Miranda and less and less the Andy her friends used to know.
From the time after her make-over, Andy made a series of affirmations of a particular sense of appropriate and inappropriate behavior, of a particular understanding of what constitutes right or wrong strategies, of particular stakes of the game or values.
Her no at the very end could be accounted for by the fact that her embodied sense of what was appropriate and inappropriate (i.e. her sense of what was appropriate and inappropriate before her make-over), her sense of the rightness or wrongness of strategies and most importantly, her sense of what stakes were worth playing for were too different from the sense that the fashion industry made her swallow.
It is important to note, however, that it could have gone either way. She could have, at the very end said no to everything that she was before. She could have gone on and said no repeatedly to her boyfriend and her friends and her sense of style and deportment before her make-over. She could have fully embraced her lover and her job and her new lifestyle.
To say yes to something different is difficult, yes, but not completely impossible. Our sense of what is appropriate and inappropriate, what is the right or wrong strategy, and most importantly, what stakes are worth playing for are "durable but not eternal".