Answer:
C
Explanation:
The mention of the change of plans was most likely intended to emphasize that the nature and scale of the disaster was of historic proportions and warranted being addressed during the State of the Union speech.
While the passage does not say if the whole State of Union address was postponed or not, what is clear is that the President goes on to talk about the Challenger disaster and most probably dedicates the entire speech to the fateful incident.