Read Thread: New Doctor
I think the spaceship shown in the trailer may just be one that the group ends up on. I honestly feel that the little blue box is too much of an icon for them to mess with to the extent of fixing the chameleon circuit. :-)
The Sixth Doctor did try fixing the chameleon circuit, but it didn't last long:
I think that the Fourth Doctor did too, but I don't have the video proof like you provided.
I'm sure they wouldn't get rid of the TARDIS's exterior. It's so iconic the BBC successfully won the copyright for the design from the police. They know they would have a fan riot on their hands.
I can understand your worry though--I would be pretty upset if they changed something so fundamental to Doctor Who! (I even get attached to iconic aspects of individual Doctors. I loved Matt Smith's regeneration scene, but when he pulled off the bowtie and just dropped it on the floor, I completely lost it. I had managed to hold the tears back until that moment!)
I try not to pay too much attention to the Sixth Doctor. He makes my head hurt. LOL
He attempted to in Logopolis, even measuring the exterior's "37 dimensions" with Adric, however it failed when The Master began to interfere with the stability of N-space.
You may also be thinking of the secondary console room that shows up in a handful of scenes in seasons 14 and 15, including Sarah Jane's farewell.