You have to do the "ALTER TABLE..." statement and all other statements (DELETE, UPDATE, INSERT) in the same Unit of Work (UOW). A UOW is, in DB2, basically from one commit/rollback to the next. It looks like you are executing you commands from the CLP. This has autocommit ON by default, which commits each statement. You need to turn autocommit off using:
UPDATE COMMAND OPTIONS using C OFF
You will also have to manually issue the COMMIT statement when you are done.
Andy