Changing the WHAT column of DBA_JOBS
Really? DBMS_JOB has a procedure called CHANGE.

declare
  jobno number;
begin
  dbms_job.submit(jobno, 'begin null; end;');
end;
/
commit;

select job, what from user_jobs;

       JOB WHAT                         
---------- ------------------------------
         1 begin null; end;    

begin
  dbms_job.change(1, 'begin null; commit; end;', null, null);
end;
/
commit;

select job, what from user_jobs;

       JOB WHAT                         
---------- ------------------------------
         1 begin null; commit; end;