From the viewpoint of, say, cosmic rays emerging from the sun, these slow neutrinos can change helicity. A left-handed neutrino then changes into a right-handed one.
Now, the weak interaction affects only left-handed particles and right-handed anti-particles. Does this mean that a left-handed neutrino, as seen from an observer at rest wrt co-moving coordinates, participating in a weak process, cannot participate in the same process, as seen from the frame in which high-energy cosmic rays are at rest and from which the neutrino seems right-handed? How is the chirality of the weak interaction conserved? Are non-relativistic neutrinos behaving like, say, electrons, which can have both right-handed and left-handed versions but still behave chirally?