The actual text from the books is open to debate on whether the LOTGK is Jadis or not. If you are taking the narrative from the books, it makes much more sense that it is here, than not. I love the interpretation where Jadis is the Lady of the Green Kirtle. It gives much more weight to her character.
It would be much easier for the filmmakers to take this interpretation as well. They've already established the character and the fact that, though she was "killed" in LWW, she is obviously still around in some form or another and is trying to reclaim Narnia. Introducing a virtually identical character with similar motives and yet establishing the difference b/w the two, all the while trying to tell the rest of the story, would be a huge headache for the creative team, not to mention confusing for the audience that has never read the books. Making Jadis the Lady of the Green Kirtle would be, as they say, Easy-Peezy-Lemon-Squeezy.
I vote for the triumphant return of Tilda!