declare
    object_exists exception;
    pragma exception_init (object_exists , -01430);
begin
	execute immediate 'ALTER TABLE RUTA MODIFY (PREFIXO VARCHAR2(20))';	
    exception when object_exists then null;
end;
/
declare
    object_exists exception;
    pragma exception_init (object_exists , -01430);
begin
	execute immediate 'ALTER TABLE RUTA MODIFY (PREFIXOAUXILIAR VARCHAR2(20))';	
    exception when object_exists then null;
end;