I think it's a touchy area, and in the situtations where it's come up, I've always asked and been told they would cover anything necessary if it came to that that I couldnt' switch. The one time I did have to move things around I was fully insured for change plans for any reason for 90% of the cost, so it was minimal expense.
The first class ticket shouldn't matter one way or the other, it's what you lose by not taking the trip because of work, so it's what they should cover if they are going cover it. If they are requiring you to be there, and it's their choice, then they should cover everything no questions asked.