"The cub at the end was/is a male/Kopa because the official books say so!!!"
NOT.

1. Most of the official books from 1994 state it's a boy, but it matters not because this book is equally official and states it's a girl!

2. The official book "A Tale of Two Brothers" wherein Kopa first appears, tells a story of Rafiki arriving to Pridelands for the first time when Mufasa is already a teenager. But this book this book is equally official and states that Rafiki had met Mufasa when Mufasa was a cub. As the last mentioned is equeally possible truth and contradicts Kopa's family history, the cub in the film is not Kopa.

3. The official book "A Tale of Two Brothers" wherein Kopa first appears, states that Mufasa's father named Zazu's mother and every future generation of her family as the stewarts to the kings of Pridelands. But the equally official book "Friends In Need" states that Mufasa personally named Zazu as his stewart after he personally remarked the bird's usefulness. As the last mentioned is equally possible truth and contradicts Kopa's family history + the cub could as well be a girl in its story - the cub in the film is not Kopa.

RESULTS IN: The book universe supports the film's creators' genderless-Fluffy vision. (In the 1993 movie script it was refered to as just "THE CUB" with no gender reference, and still in 1995 and 2003 the film's creators called the cub a unisex nickname "Fluffy" and made no gender reference.) Also Simba's Pride's makers have officially confirmed that in their story Fluffy is Kiara. In this official interview they say "It starts off where the first film left us with a twist - you think it's a king but it's a queen!" And remark also that he says that we THINK it's a boy, not that it actually was in the film itself.

SO, the cub is in fact:

- A BOY AND A GIRL in the book universe.
As in the books and the movie(s) are seperate entities because one character can't have two genders in one entity.

- A GIRL in the entire movie universe storyline which includes The Lion King and The Lion King 2: Simba's Pride.
As in Kiara because TLK2:SP's official trailer blends the cub from the end to Kiara and again one character can't have two genders/identities in one story.

- GENDERLESS in the purest canon (the first film) because its creators refered to it in utterly unisex way in everything; the script, the movie, the commnentary track.
As in it is whichever gender a fan prefers, becoming your original character and not a canon gender.

You're entitled to have, state and keep your opinion that the cub was/is a boy in the film too - but PLEASE stop claiming that your opinion is also an official fact. Because it's not. And in the end whether you think it's possible or not for it to be a girl, it is anyway, as some official facts have turned it into a girl.

In fact, it's a girl more officially than a boy because it's been stated a boy only in the book universe and never in the movie universe, whereas it's been stated a girl in both, the movie universe and the book universe.

Thus, Kiara did not replace Kopa, Kiara did not replce a boy, the gender was not changed; because in the movie universe the gender simply did not officially exist until Kiara was created.