If you have an SNMP write community set, you can also overwrite the password.
1.3.6.1.4.1.25053.1.2.1.1.1.1.25.0 sets the username, and accepts a string.
1.3.6.1.4.1.25053.1.2.1.1.1.1.26.0 sets the password, and accepts a string.
Chances are, though, that if you don't know the password you won't know the SNMP community, but it is worth knowing in case you do.