Here's why Lavender Brown was recast in the Harry Potter film series. The character, a notable member of Dumbledore's Army in the book series, was portrayed by three different actresses over the span of the eight movie installments, with the most notable being Jessie Cave from Harry Potter and the Half-Blood Prince onwards.
Lavender Brown was a fellow Griffyndor alongside Harry Potter, Hermione Granger, and Ron Weasley. She started at Hogwarts the same year as the trio and was often present in many of their classes. The young witch studied Divination and was a loyal supporter of Professor Trelawney. Lavender was later one of the original students who joined Dumbledore's Army, an organization meant to stand up against Dolores Umbridge. The group would go on to play a key role in the final battle of the Second Wizarding War.
Actress Kathleen Cauley portrayed Lavender Brown in Harry Potter and the Chamber of Secrets. The character was then played by Jennifer Smith in the following movie, Harry Potter and the Prisoner of Azkaban. The role of Lavender notably went through yet another change in the sixth film, Harry Potter and the Half-Blood Prince with the casting of Cave. So just what was the cause for the major recasting?
The simple answer is that Lavender Brown's role becomes more prominent in the sixth Harry Potter movie. As such, Warner Bros. decided to go with a more established actress in Jessie Cave. The previous appearances of the character in the early films were non-speaking roles appearing in a few scenes as a background character; Lavender was never referred to by name before Cave took over the role, so it's even possible the casting director assigned the name to keep track of the minor roles while filming.
The recasting did, however, came with some controversy due to the two previous Lavender actors being people of color. Many fans have accused the franchise of whitewashing in this situation as they could have cast an experienced black actress in the expansive role. Others argue that Cave's portrayal was based on the book's description of the characters, although Rowling never actually stated Lavender's skin color in the books.
From a Harry Potter story standpoint, Lavender lived a few happy years as a student at Hogwarts, including the short-lived fling with Ron (much to the dismay of Hermione). In the end, though, she didn't get the future she had hoped; Lavender was killed during the Battle of Hogwarts while protecting her school and everyone inside the walls of that castle. As for the actors who portrayed her, while Cauley and Smith are no longer acting, Jessie Cave has had a successful career on stage.