Normally I am myself in all my dreams. I only remember one occasion where I was someone else in a dream. It became my fondest nightmare. When I was a middle aged man I had a dream in which I was a teenage girl. There was no sense of surprise or disbelief with this situation. I was a girl with no awareness at all of my male identity. This girl came complete with a background history to round out this sense of female identity and personality.
I was just curious how common this might be. It only happened to me once that I am aware of.