Is shifting the narrator's voice cheating?

The story I just finished used a technique that happens around here fairly frequently, a first person narrator telling the story in the past tense. And as with many pieces, my narrator starts out male and becomes female at the end. But as I was editing it, I noticed that the voice of my narrator starts out from the point of view of a man, and gradually shifts to that of a woman. I didn't want to do a rewrite, since it seemed to work, but isn't it cheating, in a way? If the story is being told in past tense, shouldn't that mean my main character has already become a woman before she started telling her story, so starting it out with a masculine tone to the narration is wrong?

How do the rest of you feel?