I did that, too, with Gemma. We really thought she was a boy (2 dark dots on her fur in a strategic place were thought to be testicles. I could only get furtive glimpses, as she was feral, and wouldn't let us touch her!). I named her "Jack". Imagine my amazement when, after her 1st vet appointment, the receipt had "Jacqueline" on it!

(I've since changed her name to "Gemma".)