Oracle 11 passwords and upgrade

In Oracle 11 passwords are case sensitive by default. When you are upgrading older database to Oracle 11 password remains case-insensitive but if you change passwords they are starting to use case sensitivity.

You can check PASSWORD_VERSION column from DBA_USERS view. If users password version in there is lower than 11 then it is not case sensitive but if it is eleven or above then it is case sensitive by default. And if there is both 10g and 11g in column it points out that password assigned to user in 10g but then upgraded to 11g (case sensitive):
SQL> SELECT username, password_version FROM DBA_USERS;

If you want to take case sensitivity of you can do it with setting FALSE on "sec_case_sensitive_logon" parameter:
SQL> ALTER SYSTEM SET sec_case_sensitive_logon=false ;