I had a similar fantasy. I went down an elevator, and got younger and younger until I was a single cell. Then the XY chromosome in that single cell was switched to XX. Then I went back up the elevator and got older and older until I reached ground level. I became the female version of me.
Another one that I liked was that somehow I got locked in a department store and couldn't get out. So, I decided to stay until the next morning and try on whatever clothes struck my fancy.