Originally Posted by expert7700

Does your business policy allow business cabin for travel over X hours? Just asking since it would further the cause that the agent made a mistake in not disclosing.

More than likely , the agent didn't even notice it themselves. They were expecting a difference in price, told you a price, and issued the ticket when you paid.

As posted above, if a TA didn't clarify the change, just asked for x dollars to change a date, I'd escalate and ask to be moved back to your original class of travel, at no additional cost to your company. If they push back, ask them to completely unwind the mistaken change, by refunding you the $ and putting you back into P class the day before.