Here's why:
* Visual difference: The wardrobe and makeup for Gambon's Dumbledore were deliberately distinct from Harris's portrayal to avoid a direct imitation. This helped to establish a separate identity for Gambon's Dumbledore and marked a new era in the films.
* Age and appearance: The visual change also reflected the natural aging of Dumbledore, as the character's appearance is supposed to change over time. Gambon's Dumbledore was portrayed as older and more weary, while Harris's version was more vibrant and energetic.
* Costume differences: There were several visual differences in the costume design. Gambon's Dumbledore wore a more muted-colored, more traditional-looking robe, while Harris's had brighter colors and a more flamboyant style. Gambon also had a different beard style, which contributed to the overall difference in appearance.
Although the filmmakers made a visual distinction, Gambon embraced the spirit of Harris's portrayal and infused his own interpretation of Dumbledore, drawing on the foundation laid by Harris.