Maybe some ability from a Legendary causes it (since Zygarde is already used, it's exempt)? In these choices, the girls still have their goals, unless the conflict with Ash's morality (so, for example, Ash in a villainess's body won't work for an evil team), fashion sense, Pokémon type preferences, and if they didn't love Ash before, they will.
Choice 1: The Lake Trio, upon being freed from Cyrus, combine their powers to give Ash an ability. If Ash puts his right pointer and middle finger on a girl's forehead, and channels his aura, he can rewrite her will, emotions, and knowledge to make her a copy of him, but with her memories as well. A link of a red chain will appear on her forehead, then the girl will pass out for five to ten minutes as the new Ash chooses personality traits from the girl to keep. The girls' eye colors will change to Ash's, but only Ash, Dawn, and Brock will be able to notice the chain links on the foreheads or eye color changes, and in pictures and reflections, they'll look normal. Ash can also alter emotions of girls and alter memories of others, both boys and girls (he can't create new memories, but he can erase and restore memories, and if he erases them, memories relating to knowing these things will be altered) to do either of these alone, he must channel aura while making eye contact. The Lake Guardians tell Ash, Dawn, and Brock this in an astral plain where time doesn't pass.
If Ash does the same, but with the fingers on his left hand, he turns then into Brock, but with a green chain link. An altered girl can't be changed to be the other boy. If he puts his left pointer and middle fingers on Brock's forehead, the copy mind he has stored for Brock will be updated, and all future Brock copies will have the most recent update.
Ash will get Mars and Jupiter alone, turning Mars into an Ash, and Jupiter into a Brock. Later, when they're alone, Dawn asks Ash to turn her into another Ash (Dawn secretly wanting this is why they let her know. Ash doesn't turn girls more than two years younger than Brock into other Brocks. He also won't alter married women (Ash's mother is still married, but Grace and Johanna will be treated as widow, and since everyone thinks Lusamine's husband is dead, he won't mind changing her). No having s mother-daughter sexual relationships.
Choice 2: Reshiram sees what Zekrom did to Pikachu, and decides to even the scales by giving Ash a power of truth. It combines with Ash's aura. If Ash lays one of his palms on a girl's skin and channels aura, then within five to ten minutes from then, the girl will start heating up, showing the symptoms of a fever, and will need to rest. Upon falling asleep, she won't wake up for eight hours as a new truth, that she's Ash in the girl's body, burns itself into her soul.
Choice 3: When Ash sees Ho-Oh, it is able to recognize him as one with a great destiny, and bestows an ability. Ash can produce an aura infused Sacred Ash from his pointer finger. If he draws the symbol on his hat on a girl's forehead, she will turn into ash, which will collapse on the ground. Her body will immediately reform, with her being Ash mentally. Her hair or breast size may be increased. Only her body (including her hair) will turn to ash, not her clothes, Poké Balls, etc, so she'll reform naked. Any of her Pokémon will treat Ash like they would her.