SNMP v3 user chg pwd

Hi,
I can create snmp v3 user by editing snmp.conf
 
rwuser initial
createUser initial MD5 setup_passphrase DES

 
I tried to use snmpusm to change the password of initial and I got the
following error msg
 
#>snmpusm -v3 -u initial  -l authNoPriv -a MD5 -A setup_passphrase  -x DES
-X setup_passphrase  localhost passwd setup_passphrase  test1234 initial

Error in packet.
Reason: inconsistentName (That object can not currently be created)
Failed object: SNMP-USER-BASED-SM-MIB::usmUserAuthKeyChange."........."."
initial " 
 
Any idea what may be wrong?
 
What should I do in order to change the user initial password from
setup_passphrase to something else?
 
Thanks
 
Connie