as others have said, that would confuse the audience if it was Tilda Swinton, because, and this has been pointed out as well, the WW and the GW are not the same exact person. the Green Witch is of the same kind as the WW. Meaning, they're not exact, but similar. Which also means, someone who can act similar to Tilda but not copy her performance; someone who's similar to her in appearance, but also different.
Tilda would just confuse the audience and if the filmmakers stay true to the spirit, they'll get someone who can pull off a very similar performance given by Tilda.