For CX Economy fares ex-USA, change fee depends on fare family, not booking class. As long as the ticket is in ECONFLEX (the 3rd and 4th code of fare basis is "41"), there is no change fee. If the corporate agency needs to book the lowest class but can issue the more expensive ECONFLEX fare, the ticket can then be upgraded to Premium Economy by just paying the change fee (if the agency does not charge another fee). Otherwise, I am not sure if the USDOT rule can be cited for free changes within 24 hours after booking, or if the agency will need to refund and issue a new ticket.