I have seen a few renders of male characters made into female characters.
mostly spirit of course made into a mare for a person's fan character. Most are pretty good, really good. However i have a few pointers.
1.) change the eye shape if you can the mare's eyes in the film seem to be longer, thinner, like this symbol ~ and it's slightly tilted.
2.) If you can't change the eye completely always as lashes and a eyelid line, it makes it look feminine.
3.) Make the slope/bridge of the nose curved up slightly. the stallions in the film have straight noses, were as the mares noses are curved up slightly, and there nosterals are a bit slanted as well.

Do you see the difference between rain's nose and spirits?
4.) this isn't a must but i have noticed that the mare's jaw is a bit bigger than the stallions jaw in the movie as well this may or maynot help with your render.
anyway i hope this helps and i hope i put this is the right section.