One alteration to a character's backstory in the "Harry Potter" series that could change the entire plot of the movies would be adjusting Severus Snape's motivations and allegiance.
In this alternate version, Snape is still a former Death Eater, but his loyalty to Dumbledore and the Order of the Phoenix is less driven by his love for Lily Potter. Instead, Snape is motivated by a personal sense of justice and regret over his past actions as a Death Eater. He joins the Order out of a deep-seated desire to make amends for the harm he caused. Without the love-based motivation for protecting Harry, Snape's relationship with Harry and Dumbledore becomes more complex and ambiguous.
Below might be the plot Impact:
Snape's Relationship with Harry: With his connection to Harry's mother removed, Snape might treat Harry more harshly or be less protective of him. The evolving relationship between Snape and Harry would reflect a gradual shift from hostility to respect based on Snape's commitment to redemption.
Snape's Loyalty to Dumbledore: Snape's allegiance to Dumbledore would be more focused on their shared goal of defeating Voldemort, without the personal aspect of his love for Lily. This could make his interactions with Dumbledore more business-like and less emotionally driven.
Role in the War Against Voldemort: Snape's sense of justice and redemption might lead him to take more active measures against Voldemort's followers. He could play a more visible role in the resistance, engaging in direct confrontations with Death Eaters.
Snape's Sacrifice: Snape's sacrifice at the end of the series would be based on his commitment to a greater good rather than a personal promise to Lily. This could make his final moments more focused on his journey from darkness to redemption, giving him a more prominent arc of personal growth.
By changing Snape's underlying motivations, the plot takes on a more complex tone, emphasizing themes of redemption and justice rather than solely focusing on personal loyalty and love. This adjustment could lead to new character dynamics and alter the interactions between key characters in the series.