His core personality might not change much. Luffy is still likely to be as carefree and adventurous as ever. However, he might become a bit more sensitive to certain things, like how people treat girls. He'd still be fearless in the face of danger, but could use his new form to his advantage in different ways, like when dealing with female characters who might be more likely to trust a girl.
Luffy's personality would likely remain at its core the same. He'd still be the goofy, food - loving pirate. But, he might start to notice things from a different perspective as a girl. For example, he could become more aware of the power dynamics between genders in the pirate world. He might also use his new identity to befriend different groups of people, especially those who are more open to a female presence. His determination to reach the One Piece wouldn't waver, but his approach to achieving it could be influenced by his new form.
Even as a girl in the fanfiction, Luffy would probably keep his positive and exuberant personality. He would still laugh in the face of danger and be ready to take on any opponent. However, he could develop a bit more empathy towards the female characters he meets. His loyalty to his crew would be unshaken, but he might show it in different ways. Maybe he'd be more of a mediator within the crew in case of disputes, using his new - found understanding of different perspectives. His love for adventure would continue to drive him forward, but his interactions with others would have a new layer added due to his changed gender.