Recently, I needed to update the SNMP settings on some old MacOS X file servers that I was monitoring. There was no GUI to change the SNMP settings on those, only to start and stop it. Since I was using the command line to reconfigure lots of other servers and switches, I didn't want to resort to the GUI just to restart the SNMP process on these Mac servers. This is what I ended up doing.
First, I made a new snmpd.conf file the Mac in the usual way.
Then I put the new file into place and restarted the snmpd process.
sudo cp snmpd.conf /usr/share/snmp/snmpd.conf launchctl stop org.net-snmp.snmpd
Technically, that only stopped the process. However, the org.net-snmp.snmpd.plist configuration file states that snmpd should always be running. So the launchd process in MacOS just starts it back up when it sees that it isn't running. It happens so quickly that a "stop" command is practically a restart in this case.
Its worth noting that these are older file servers running MacOS X Server 10.6.8. I don't know if the launchctl command would be the same in newer versions.