If you have ever played TS3, making a sim involves playing with the sliders to create the look of their face. Changing the gender in TS3 would keep all the sliders in the same place but make them the opposite gender. It works the same way in TS4 except for we don't have sliders. Same looking sim, just the opposite gender. All the genetics stay the same. I hope that made sense.
You can always save that sim to your library before making the change and/or make a backup of the save. Just incase you regret it later.
Back up Save as > Example Save 2
Library Save to bin from the option bar above while in CAS (folder icon I think?)
