Buffy dresses as Paige Matthews (Halliwell) with The Book of Shadows for Halloween. She becomes ½ Witch and ½ Whitelighters. Once the chaos spell ends she remains ½ witch and ½ whitelighter. She keeps all the abilies, I.E. Orbing, healing hands, Wiccan magic, and gained a magical core so she can use wand magic once she learns it. She gained height so she is about 5'10", and has red hair, its darker than the characters however.
As the slayer essence is from demonic origin she no longer is the slayer as it can't coexist with her being a whitelighter. However the conditioning and fortification the slayer essence has made to her body remains, meaning she remains stronger, faster, heals quicker. But only the conditioning, all the innate knowledge of weapons and the precognitive dreams are gone. She keeps her skills with the weapons and martial arts she has already used and learned, but will not gain new knowledge unless she learns it the normal way.
Buffy hides her changes under a glamour tied to a ring.
After Halloween she distances herself from Angel slightly, and he takes it negatively. He ends up sleeping with someone else and looses his soul earlier. She still argues with her mother, and still gets kicked out. Instead of staying in LA she ends up in England. She stumbles across the Leaky Cauldron and checks it out. She ends up getting a job helping Tom the Barkeep. She visits the Alley and Gringotts. Decided to take the name Paige Warren and gets I.D. through Gringotts.
Eventually she runs across 4 year old Harry Potter who sees her, and shouts "MAMA!" and clings to her leg. Petunia jumps at the chance to get rid of "The Freak". Buffy takes Harry to Gringotts and Blood adopts him. Unknowingly ends up adopting Harry as his "father" due to the Blood Protection from Lily. Side-effect is Harry becomes female. Harry takes the name Pellagri Lily Warren. Buffy starts learning wand magic. It becomes apparent through accidental magic that Harry (Pellagri) has whitelighter and wiccan abilities from the Blood Adoption.