If I understand correctly, conservation of baryon and lepton numbers imply that quarks and leptons are "basic" i.e. non-interchangeable particles? What happens when one such particle is annihilated, can the energy produced be used to "generate" the other type, or do some additional particles always "carry away" the baryonness or leptonness? In a grand unification of the strong and electroweak forces, would it not require a new symmetry that does allow these particles to interconvert, i.e. a failure of conservation of those numbers? Is that the presumed mode of a spontaneous proton decay, the thing everyone has looked for unsuccessfully so far? If this topic has been discussed previously please direct me to the relevant post. I tried a text search but could not find something exactly appropriate. Thanks!