If she changes her major after initiation, nothing happens. She is still a full member of Alpha Omega Epsilon.
Nothing even happens to her membership: she is still entitled to all rights of an Active-Active or Alumnae-Active.
In my particular chapter, we've had English majors before, a finance major, and a psychology major. They all switched their major after freshman or sophomore year and chose to remain active still.
I'm not sure of the reasoning behind it, but I'm glad it exists: it would be so sad to lose a sister you've come to love.
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On a different note, my best friend from high school is now an SAI! I sent her red roses