[DTV]

I think they were meant to be in B today but due to the camera issues didn't bother.

This does definitely seem like the case now, there's quite a bit of circumstantial evidence pointing that way. Also pretty much confirms that D is being readied for something else, with the fact it was in a transmittable state suggesting it's not long before it is put to that use.

Why would they move to Studio D at quick notice, a studio that has been changed since they last used it. It would have made more sense to go back to Studio A where they have been for a few months

If it is the case that D's handover to its new user is approaching, it might have been that they were piloting in D anyway and so it made more sense to continue in D rather than fire up A.

It could also be the case that, given the Mayoral interview was clearly something special, that they intended to have it in the new set. The team may have decided that it was better to have it in at least a new set, particularly if it had been communicated to the Mayor's team that way.