I've been problem when I tried to change the Internal/Sys user password
Internal is not exactcly same as SYS. The password file is meant for INTERNAL not for SYS. Let me explain it with an example:

You have the default password for SYS change_on_install. Then you connect as SYS:

$sqlplus /nolog
SQL>connect sys/change_on_install
Connected.

Now create the password file:

$orapwd file=orapwABC password=xyz entries=3

Now, try again:

$sqlplus /nolog
SQL>connect internal
Connected.

Startup the instance

SQL>connect sys/change_on_install
Connected.

connect sys/change_on_install as sysdba
Now you get ORA-01017: invalid password

The difference is that SYS connecting as SYSDBA is like INTERNAL, they both use the password file, SYS however, uses its own password.