snmpusm with -Ca , -Cx option

Hi ,
 
 I am creating SNMPv3 users using snmpusm. But I am not able to change
the authentication key using -Ca option.
 
Using the below command I am able to change password for user testuser.
snmpusm -v 3 -u testuser  -l authPriv -a MD5 -A "nobody knows it" -x DES
-X  "nobody knows it"  localhost passwd "nobody knows it" mysecretpass
 
How to use -Ca and -Cx options to change the authentication key and the
privacy key ??.
 
 I am trying with 
snmpusm -v 3 -u testuser  -l authPriv -a MD5 -A "nobody knows it" -x DES
-X  "nobody knows it"  localhost -Ca passwd "nobody knows it"
mysecretpass
 
Thanks in advance
 
Regards,
Sailaja