According to another article I since read, an additional reason for the move is the hilly nature of the exhibit at Pairi Daiza, which the two older elephants are starting to have some issues with.
Pairi Daiza stopt met Afrikaanse olifanten: verblijf is ongeschikt
Although Pairi Daiza has not talked about it yet, I wouldn't be surprised if they used the current elephant exhibit to expand their mixed savanna exhibit. I believe they once intended to allow some of the savanna animals access to the elephant area anyway, but that that never worked out. With the possible addition of eland extra space for the savanna animals would be welcome.
It seems the obvious way to use this extra space. As you said, if they get more Eland to keep the 'rescue' one company, they would need space for them too. Perhaps add a gazelle species or something else smaller for contrast with the larger animals as well?