Error I'm getting: ORA-16003 standby database is restricted to read-only access

I'm trying to change the password for SYS, SYSTEM, and some others on an 11g standby database. I've tried:
alter database open read write;
ORA-01531 a database already open by the instance

I've also tried:
shutdown immediate;
startup force open read write;
alter user SYS identified by <password>;
ORA-00604 error occurred at recursive SQL level 1
ORA-16000 database open for read-only access

I've also tried using FORCE, but that didn't work either.
TIA!