I was moving my NAS to a different network. My FreeNAS server had IP 192.168.2.5 and has now 192.168.4.5 insteed. I was not able to make changes in the web GUI, SMB section because it could not find the interface for 192.168.2.5. The drop down menu gave me only a "-" to choose from.
Work-around: I was able to save my configuration file and then edit it with hexedit. Searching for ASCII "192.168.2.5" and change it to "192.168.4.5". Repackage my 2 files into a .tar file and then upload my new configuration. Problem solved.