The interior of the Doctor's fancy flying time-box is getting an overhaul for the holiday episode.
On the inside, the TARDIS is infinite, creating new rooms as needed within itself, but rarely do we see more than the current control room. (I say 'current' because the other past control rooms are all still stored in the TARDIS, just not being used).
Typically, when the Doctor regenerates, a new control room will also form, and so a change in actors also means a change in the visual style of the room that people associate most closely with the TARDIS.
There have been exceptions to this in the past of course, a couple of regenerations resulted in no change, especially early on, and there have been cases of extreme damage to the TARDIS requiring intense internal reconstruction, which effectively resulted in a new look. Then there is that time that Tom Baker's Doctor spent half a season hanging out in a 'secondary control-room' that looked like a Victorian tea-chamber.
Well, we'll be seeing another one of those relatively rare exceptions in the next episode. When the holiday episode airs in a few weeks, a new TARDIS design will arrive as well, though Matt Smith is remaining in the role of the Doctor. We have no word yet on what causes the redesign canonically, it might be damage to the TARDIS or the Doctor's mood swings - his current control-room seems very playful, but in the new episode we'll be seeing a harder, less juvenile Doctor - or we could get no explanation at all, left to assume that something happened in the months or years since we last saw the Doctor.
Whatever the reason, we have a sneak peek of the redesign below:
The image looks like it's of the top of the control column in the center of the room, around which the confusing controls are all arrayed. Those symbols seem to be Gallifreyan language. The nice, regular angles and recessed lights indicate a certain sleekness, a welcome change over the frenetic and busy design that we've had for the last couple seasons.
"It’s sort of like getting a brand-new sports car." said Matt Smith on the Doctor Who production blog, "It’s always a bit exciting."
The second half of this series (season) of Doctor Who kicks off in the new year, but in between we'll have the holiday episode which is slated to air on December 25, 2012. A Doctor Who 50th Anniversary special will also arrive sometime in 2013, but the series will be on hiatus after that until Fall 2014, when series 8 is scheduled to be split across the new year, similar to this season.