I was going to say I wasn't sure, but double checking, I think you may be right. I assumed because of the different background that the black-and-white section of the trailer was just Bradley footage altered to look vintage, but double-checking and doing a side-by-side comparison, it looks like what's actually going on is that for the black and white section, they took Hartnell's head from the original scene and put it on Bradley's body in the new footage. That seems like a lot of work for a trailer if you're not
that guy on YouTube, so it's possible that transition actually will be in the special, so they might be planning to go out on the same device. I'm not sure, though.
It would seem to me like putting a hat on a hat, and it's hard to go back and forth between black and white and color. If I were going to do it, I'd have to have the episode begin with Tenth Planet footage, go to color once, and then only go back to black and white at the very end, which is a problem because it makes a lot more sense to end on the new Doctor... that is, on the new Thirteenth Doctor, rather than the new Second Doctor. It'd be weird to go back to pillar-boxed black and white, and then jump back into HD living color for Capaldi's final scenes and regeneration.
All in all, I'd prefer an all-new redo of the regeneration scene instead of just a tease. That still wouldn't require a Second Doctor, he's not moving, they can just use a photo, or bust, or digital double. They did it with Eccleston. Twice!