They were probably treating it as a normal ticket change, which they're not supposed to do with agency-booked tickets. Same Day Confirmed Changes are different though -- they cost less and are designed to be done only at the airport. Travel agencies can't do them, and that includes the VX call center. Once you're at the airport, the ticket is in the airlines hands and they can do what they want. If you're flight got cancelled, for instance, they would rebook you and be able to change it themselves. They wouldn't ask you to call Chase. The same logic is true for SDCs.