Fixpack Downgrade failure in Db2 version 11.5

Summary

On a certain Db2 version which are installed on systems, fixpack downgrade may fail with either 'installfixpack' or 'db2iupdt' commands.

(installfixpack)
# ./installFixPack -L -b /opt/IBM/db2/V11.5_EAP3/ -p /opt/IBM/db2/11.5.0.0.0.1 -f level -l /tmp/db2fix_by.log1 -t /tmp/db2fix_by.trc1
Downgrade to "11.5.0.0.0.1" from the current level "11.5.2.0.0.0" is not supported.

(db2iupdt)
# db2iupdt -f level xxxxxxxx
DBI1446I   the db2iupdt command is running.

..

For more information see the DB2 installation log at "/tmp/db2iupdt.log.30296".
DBI1264E  This program failed. Errors encountered during execution were
             written to the installation log file. Program name:
             db2iupdt. Log file name: /tmp/db2iupdt.log.30296.


# cat /tmp/db2iupdt.log.30296
..
ERROR: Downgrade to "11.5.0.0.0.2" from the current level "11.5.2.0.0.0" is not supported.
ERROR: An error occurred while configuring the instance "db2inst1".

Configuring DB2 instances :......Failure
Updating global profile registry :......Success


This is due to a defect in earlier build of DB2 V11.5 versions.
Even with special build that includes the fix for the defect, the same error may still happen because of running commands on the version that is supposed to be updated from.
As there is no way to manually edit this executable as it is already in binary form.