Neutrino-oscillations motivate the search for new beyond-the-Standard-Model particles that might manifest as light scalar bosons mediating lepton-flavor-changing (LFC) interactions. In a certain class of theories, the light scalar boson could be produced at accelerator neutrino beams, where a high-intensity, high-energy muon flux is expected. An example of such a facility is the Deep Underground Neutrino Experiment (DUNE) using the high-power proton beam at LBNF. If such a particle exists, it could be produced after the DUNE LBNF beam target. If it is long-lived, it could travel a few hundred meters, and it could be detected at the DUNE Liquid Argon Near Detector. In parallel, the existence of such a new LFC scalar boson can also be tested using the 160-GeV muon beam at the fixed target NA64mu experiment at CERN, in a complementary way. In this talk, light long-lived LFC boson signatures and projected sensitivities at DUNE and NA64mu will be presented.