It could have a major impact. Harry's relationship with his teachers might also shift. Snape, who caused the transformation, might have some unexpected reactions to seeing Harry as a girl. Other teachers might treat 'her' differently too. And as for romantic interests, well, Harry might attract different people now. Some boys who never considered him before might start to notice 'her', which would create a whole new dynamic in his social life at Hogwarts.
In some fanfiction, this could happen through a powerful spell gone wrong. Maybe Snape was attempting a complex transfiguration spell for some secret reason, like protecting Harry from a dark magic threat in a very unorthodox way. And the result was Harry being turned into a shifter.
Well, if Snape turns Harry into a vampire in fanfiction, it would completely change the dynamic. Harry would have to deal with new instincts like the thirst for blood. His relationships would be strained as his friends might be scared of him. Also, his role as the 'Chosen One' would be redefined as he's now a different kind of being with different needs and abilities. He might have to struggle between his human side and his new vampire nature.
Perhaps the plot could involve Petunia having some sort of magical awakening or discovery that leads her to perform this transformation on Harry. In this fanfiction, Harry might then use her new female identity to infiltrate parts of the wizarding world that are more difficult for a male to access, like certain all - female magical groups. And this could lead to all sorts of adventures and revelations about the wizarding world's gender dynamics.
Another possibility is that it's an accidental result of a powerful magic clash between Voldemort and Harry. When their magic collides in a certain way, it causes an unexpected transformation on Harry, turning him into a girl. This would add a whole new layer of complexity to their relationship and the overall story.
His relationships would likely change a great deal. Some friends might be more protective of him now that he's a girl. Others might have a hard time seeing him as the same Harry. The boys who used to just roughhouse with him might be more hesitant. And Harry himself would have to navigate new social dynamics, like how the girls at Hogwarts interact with each other. It's a big shift for all involved.
The students would have a wide range of reactions. The Gryffindors might be more vocal about their surprise, with some being more accepting than others. The Slytherins would be more divided. Some of Snape's loyal Slytherin students might be protective of him, regardless of his new form, while others might see it as a sign of weakness. Draco Malfoy would probably be initially disgusted but as he sees Snape still maintaining his authority, he might change his attitude. And the Hufflepuffs would generally be more kind - hearted and accepting, perhaps offering Snape small tokens of support like homemade treats or kind notes to help him through this difficult change.
Well, in the fanfiction world, if Snape turns into a baby, it could be really chaotic. His colleagues at Hogwarts might be extremely shocked. Maybe Dumbledore would have to take on the role of caretaker, trying to figure out how to reverse the spell. And Harry Potter would probably be very confused seeing his former strict professor as a helpless baby.
One possible plot could be that Snape, after being hit by a strange spell during a battle in Hogwarts. As a woman, he (now she) would face different reactions from the students. Some might be more sympathetic, while others would be confused. She would have to deal with her own changed identity while still trying to teach Potions and keep an eye on Harry Potter, all the while trying to find a way to reverse the spell.
Petunia's character could also develop in this fanfiction. If she turns Harry into a girl, she might start to feel guilty or responsible. This could lead to a change in her relationship with Harry. Maybe she would start to see Harry as more of a person rather than just a burden. And she could also develop some magical abilities further as she has now performed such a complex spell on Harry.