Just hold on a minute. This could be more misdirection from Abrams. Maybe the premiere screening has been cunningly edited to give the illusion
Harrison is Khan, convincing fans worldwide that they finally know the truth. Then, when it premieres proper to the public - bam! The final curve ball will be thrown and we will leave the theater in awe of the web of obfuscation JJ has masterminded.
Oh JJ, you clever sausage. I'm on to you! (It's gonna be Mitchell, right? Gotta be Gary Mitchell.)