Wow. Major quote fail. :/
You mean Xion going from KH1 Kairi to KH2 Kairi like Xenka said? Impossible. If Xion's appearance is being changed by Sora's memories, then how would she be able to change into KH2 Kairi if Sora hadn't even seen what Kairi looked like until late KH2?
As of right now though, she can't. While I'm not saying that she won't in the future, she's not a wielder in the canon storyline.
If she were, Riku wouldn't need to hand the Keyblade to her. She could summon it herself.
If she were, she'd be standing next to SRM pointing her Keyblade at Xemnas and shouting, 'No.'
If she were, Aqua'd be picking up her Keyblade in the secret ending and not Mickey's.
While Kairi may become a wielder in the future, in the present she is not.
Alright then, rivalry. Keep whatever it is in VMs.