Downgrading from DB2 Enterprise to DB2 Express-C is a fairly straightforward process.
Uninstall DB2 Enterprise.
sudo su - db2inst1 /opt/ibm/db2/V9.7/install/db2_deinstall -a exit
Install DB2 Express-C without creating any users, etc.
Update the instance with the new edition information.
sudo /opt/ibm/db2/V9.7/instance/db2iupdt db2inst1
You might encounter this error.
db2inst1@vhost0074:~> db2start SQL5043N Support for one or more communications protocols failed to start successfully. However, core database manager functionality started successfully.
Look up your DB2 TCP/IP service name. It should be similar to db2c_db2inst1 where db2inst1 is the instance owner.
cat /etc/services | grep db2
Make sure it’s set correctly in the DB2 configuration.
db2 update dbm cfg using SVCENAME db2c_db2inst1
You should now have a working DB2 Express-C installation with all the same databases.