Ok, in this case when using the SIP generic headers methodology to change header field in outgoing messages you need to make sure you use the current duration parameter in this case. As that will determine when the field changed will be sent in either next or all messages for the call. You may want to try GC_SINGLE_SIP_SESSION in order to have it used thru the established SIP dialog.
More information on this topic can be found in section 4.9.8.1 of the GC IP TUG:
www.dialogic.com/.../globalcall_for_ip_hmp_v12.pdf
Jeff