declare
    column_exists exception;
    pragma exception_init (column_exists , -01451);
begin
    execute immediate 'ALTER TABLE TARIFA  MODIFY (COMPOSICAO_DO_PEAJE VARCHAR2(300))';
    exception when column_exists then null;
end;
/
declare
    column_exists exception;
    pragma exception_init (column_exists , -01451);
begin
    execute immediate 'ALTER TABLE TARIFA_OFICIAL  MODIFY (COMPOSICAO_DO_PEAJE VARCHAR2(300))';
    exception when column_exists then null;
end;