declare 
  column_exists exception;
  pragma exception_init (column_exists , -01430);
begin 
  execute immediate 'ALTER TABLE AG_CONTAS_RECEBER MODIFY (CLIENTE VARCHAR2(14))';
  exception when column_exists then null;
end;
/
declare 
begin 
  execute immediate 'ALTER TABLE AG_ADIANTAMENTO_CLIENTE MODIFY (CLIENTE NULL)';
  exception when others then null;
end;