This question just occurred to me recently. Assuming the different neutrinos (electron, muon, tau) do not have the same mass, then if their energy is very low, such that they are moving non-relativistically, an oscillation must substantially change the velocity (to conserve momentum). It seems this must be the case, because any neutrino ascillation could be viewed in a frame where neutrinos are slow moving. Yet this is certainly an odd mental image - spontaneous speed change. Just checking that this is expected?