Originally Posted by
xliioper
Did you check individual flight bucket availability or married segment availability? Adding segment will change the fare from a PHL-ZRH fare to a PHX-ZRH fare (fares are not priced hop-by-hop).
While I understand that airfares are not priced hop-by-hop, assuming the OP are not making any change to the PHL-ZRH that are already booked, couldn't the PHX-PHL segment be added to the ticket as an additional "bound" and make the reservation a multi-city booking?
Alternatively, my understand is that the agent can also book a separate/additional ticket for the $400 PHX-PHL under the same PNR and not change the original ticket.