That isn't how it works though--look up partial pressures of gases. Helium will continue to escape until its partial pressure is at equilibrium with the external environment, i.e. near to zero, and the external gases will enter until they are at equilibrium with the internal environment. At that point the partial pressure of each gas will be equal on both sides.
Since the partial pressure of helium at ground level and flight level is essentially the same (≈0) there should be very little change, providing the seals are intact. Since they are very securely sealed to contain the helium, changes to the external pressure, unless extreme, should not have any significant effect.
