Mulan AU where she does get caught by the other fresh recruits while she's bathing but Mushu helps her spin it like the lake is cursed by an evil lizard demon and will turn men into women if they stay in it for too long.
From there it's not actually difficult to get the other soldiers onboard with covering up the fact that poor Ping took one for the team and got afflicted by the vagina curse, especially since it would have been all of them if they hadn't gotten the warning ahead of time. So they agree to help him cover it up, because obviously the army's not going to understand.
Shang is... tentatively glad that the men are bonding and getting along, even if they continue to be deeply weird about it.
Ling: Hey man, what's up— you've got boobs?!?!
Mulan: Uh, what boobs? Huh? Where did these come from?
Mushu: *facepalms and thinks quickly* (speaks from the shadows) I AM THE SPIRIT OF THE LAKE! BEWARE MY CURSED WATERS FOR THEY WILL TURN MEN INTO WOMEN!
Ling, Yao, and Chien Po: Oh no! The spirit of the cursed waters!
Chi-Fu: SHE'S A WOMAN LI SHANG!
Mulan: Look-
Ling, Yao, and Chien Po: WE CAN EXPLAIN!!
[One convoluted, chaotic explanation later]
Shang: ...is this why you've all been insisting we don't camp anywhere that doesn't have a lake.
Shang: and then none of you actually swim in it.
Shang: and you all keep jumping at shadows.
Shang: wait a second Ping did this happen before or after you became insanely good at fighting?
Shang: did you get better at fighting after you became a woman.
Shang: are women better at fighting than us.
Mulan: ....uh. well. maybe? no one's ever tried to find out.
Yao: [thinking very fast] y'know Captain it's just so hard to find recruits these days.
Chien Po: Real shortage of men.
Ling: Lots of women, though.
Mulan: [catching on] Without marriage prospects.
Shang: You're right, men. The spirits must have done this in order to show us that we should be recruiting women as fighters.
Mushu [from the shadows, seeing an opportunity to do the funniest thing]: EXACTLY, LI SHANG. I HAVE TRANSFORMED PING INTO A WOMAN BECAUSE YOU HAVE TOO LONG OVERLOOKED THE TRUE WAY TO WIN THE WAR.
Mulan [seeing an opportunity to get all the stories straight]: O Great Spirit, is it reversible?
Mushu: WHY WOULD YOU WISH TO REJECT MY GIFT? I HAVE SEEN YOUR HEART, CHILD, AND HAVE ALREADY ALTERED THE MEMORIES OF EVERYONE WHO KNEW YOU BEFORE YOU LEFT FOR THE ARMY. YOU HAVE ALWAYS BEEN THEIR DAUGHTER.
Li Shang: Welp, the spirits have spoken. Ping - wait is your name still Ping if you're a woman now?
Mulan: Uh. Actually, I was thinking of renaming myself. How do you feel about Mulan?
BONUS:
Mulan [climbing out of the eleventh lake the men have arranged for her to swim in]: Yeah no, it didn't work. Still got boobs. [tries to appear dejected].
Chien Po: If it makes you feel better, they're very nice boobs.
Mulan: Thanks, Chien Po.
Mulan and Mushu, somehow opperating on the same wavelength: oh haven't you heard?
