I’m thinking as I go, here, so I am numbering points for ease of reference, correction etc. 1. Neutrinos (excluding the anti- and sterile varieties) come in three flavours. 2. It is known that neutrinos have mass. 3. The masses of individual neutrinos are not known precisely. 4. Cosmological studies indicate that the combined mass of all three flavours is not less than 0.5 eV. 5. Also, the heaviest neutrino flavour cannot be less massive than about 0.05 eV. 6. It seems there is a considerable difference between the masses, with a very slight possibility (mathematically?) that the lightest could be massless. 7. The flavours in order of ascending mass are: electron-, muon- and tau-neutrinos. 8. It appears that as they travel through space, neutrinos mutate between flavours. Now for the question! When the mutation sequence is tau > muon > electron, where does the mass/energy go? Similarly, where does the energy come from when the sequence is reversed? My guess is that vacuum energy might come in here somewhere, but that would raise questions about the extent to which vacuum energy plays a part in the conservation of energy generally.