Running config vs startup config


I've got a question regarding NCM configuration download. For the time being my NCM deployment ic configured to download new running configuration once a week. Since the configuration on a switch port is changed every time the user connects and disconnects from the network (change is triggered by the NAC appliance which supports dynamic VLAN assignment) it's been decided that it would be better to download startup configuration instead of running configuration. Please tell me if and how such change can be made. Is it also possible to configure different time interval or not to download new configuration unless change has been made? Thank you in advance!