Well, like everyone is saying here: once you've seen someone else in the role, it's hard to find another subsitute.

Either way, I think she's just too elegantly pretty to play Tonks. I don't mean that Tonks or Natalia Tena aren't beautiful, but I imagine Tonks to be the young hip Auror and Kelly seems more like an elegant lady-like person, which matches the Grey Lady more. However, I suppose if you're a good enough actor, you can play an orange and be convincing so you never know...