  A neutron star harbors of order $10^{56}$ electrons in its core, and almost
the same number of muons, with muon decay prohibited by Pauli blocking.
However, as macroscopic properties of the star such as its mass, rotational
velocity, or magnetic field evolve over time, the equilibrium lepton abundances
(dictated by the weak interactions) change as well. Scenarios where this can
happen include spin-down, accretion, magnetic field decay, and tidal
deformation. We discuss the mechanisms by which a star disrupted in one of
these ways re-establishes lepton chemical equilibrium. In most cases, the
dominant processes are out-of-equilibrium Urca reactions, the rates of which we
compute for the first time. If, however, the equilibrium muon abundance
decreases, while the equilibrium electron abundance increases (or decreases
less than the equilibrium muon abundance), outward diffusion of muons plays a
crucial role as well. This is true in particular for stars older than about
10,000 yrs whose core has cooled to $\lesssim 20$ keV. The muons decay in a
region where Pauli blocking is lifted, and we argue that these decays lead to a
flux of $\mathcal{O}$(10 MeV) neutrinos. Realistically, however, this flux will
remain undetectable for the foreseeable future.
