Hallo,
You probably have discovered this by now: QSECOFR for the system is not the same as QSECOFR for DST. Unless you change the DST password to be the same as the system security officer password, they can be different.
The default password for DST QSECOFR is QSECOFR (all caps). The command CHGDSTPWD PASSWORD(*DEFAULT) will reset the password to QSECOFR.
DST passwords cannot be changed with CHGUSRPRF. To change them, see instructions in the 'iSeries Security Reference' (SC41-5302-05) under 'Changing User IDs and Passwords for Dedicated Service Tools (DST) Users' in chapter 4.

Gruss TARASIK