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One just has to look at how much Emma Watson's looks changed when she hit puberty - she looks very different in the 1st and 2nd film (a cute preteen, but not a "princess"-type beauty as a child) compared to from the 3rd one onwards - that it wouldn't be at all impossible to assume the same thing would/could have happened to Hermione as well.
In-books, her friends and others who knew her since she was eleven might not even have noticed that - we don't really tend to notice, consciously, how people whom we see every day change over the years. (On the other hand, also in-books, she did quite quickly catch the eye of a major international Quidditch star, without any effort on her side, so I don't think she was really all that plain by the time she was 15 or so.)
Same thing happened to Matthew Lewis (Neville), really - he went from a slightly chubby, cut but somewhat plain little boy to a teen heartthrob, basically, and I think the film people had to go into some effort to keep him looking "Neville" enough in the last film.
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