Well The Lion King 2 did come out in 1998 and if Kiara is meant to be a cub in this 'Lion Guard' then her original cub voice actor Michelle Horn, is much older now. She is 28, even if they did get her back she'll still sound different.
I don't care that much anyway, it's a low-budget spin-off series aimed at young children. I don't plan to ever watch it, so who voices who does not bother me.
From the preview clips I seen it doesn't even seem to have anything to do with The Lion King storyline, seems to me they just used The Lion King title and characters to bring it more money, take away The Lion King characters and title and nothing will be different.
Plus the 'Hyenas' look like the cross between a warthog and a mutated dog, but hey least they can animate. I can't.
Then again, don't know what I was expecting from a Disney Junior show.