1. A method for controlling a charging condition change in a real-time charging system including a packet processing node providing a service to at least one user terminal, a real-time charging function server for allocating quotas to the user terminal, and a QoS and charging rule management server, comprising: sending, if the user terminal requests service, a QoS and charging rule request to be applied to the user terminal from the packet processing node to the real-time charging function server; sending, in response to the QoS and charging rule request, a QoS and charging rule response from the real-time charging function server to the packet processing node; sending, if charging condition change is detected, a charging rule change request from the real-time charging function server to the QoS and charging rule management server; sending, in response to the charging rule change request, a changed QoS and charging rule, from the QoS and charging rule management server to the real-time charging function server; sending the changed QoS and charging rule from the QoS and charging rule management server to the packet processing node; forwarding the changed QoS and charging rule from the packet processing node to the real-time charging function server; and reallocating, at the real-time charging function server, quotas for the user terminal according to the changed QoS and charging rule.