The Constitution can not be changed in the part that mentions extradition. Only a new Constitution can change this, which is something that is impossible to happen in the present circunstances.
As said before, just because the citizen can not be extradited does not mean he/she can not be criminaly charged by the brazilian Judiciary. The foreigner coutry can perfectly start a criminal proceeding. If the citizen is found guilty, he/she won't be jailed in the foreigner coutry, but in brazilian prisons, which compared to american prisons make these last ones almost like heaven.
Many other countries have the same rules forbiding the extradition of its citizens. I personaly think that brazilian citizens could be extradited in certain cases, the previous Constitucion allowed extradition, but the current Constitution does not, and you can not question a sovereign decision of another country in these matters.