  Present experimental data do not exclude fast oscillation of the neutron $n$
to its degenerate twin from a hypothetical parallel sector, the so called
mirror neutron $n'$. We show that this effect brings to a remarkable
modifications of the ultrahigh-energy cosmic ray spectrum testable by the
present Pierre Auger Observatory (PAO) and Telescope Array (TA) detector, and
the future JEM-EUSO experiment. In particular, the baryon non-conservation
during UHECR propagation at large cosmological distances shifts the beginning
of the GZK cutoff to lower energies, while in presence of mirror sources it may
enhance the spectrum at $E \gtrsim 100$ EeV. As a consequence, a significant
reduction of the expected diffuse cosmogenic neutrino flux is predicted.
