Actually, it's yes and no.

A Seraphim has a Profane form which never changes. That's their physical, "human" body, and the Seraphim never knows what that body is going to be until it Incarnates. Their Divine form, on the other hand, is malleable and can be made to appear in any way they wish, though it is generally unquestionably supernatural. As I read it, they can't pass unnoticed among humankind in their Divine form. This information is on page 254-255 of the book.

I suspect that many Seraphim learn what "gender" they are after they Incarnate for the first time in their Profane form. Thus, all the references to the archangel Michael would be from before the archangel first took a Profane form. Once she did, she learned that she was female, and became Michele.
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You fell for one of the classic blunders! The first of which is never get involved in a land war in Asia. But the second, and only less well known, is never get involved with a sicilian when death is on the line! BAH-HAHAHAHAHAHAHA(THUD!)
-Vezzini, The Princess Bride