The long lifetime of lanthanide emitters can present a challenge for conventional pump-based modulation schemes, where the maximum switching speed is limited by the decay time of the excited state. However, spontaneous emission can also be controlled through the local optical environment. Here, we demonstrate a direct modulation scheme enabled by dynamic control of the local density of optical states (LDOS). Specifically, we exploit the LDOS differences between electric and magnetic dipole transitions near a metal mirror and demonstrate that rapid nanometer-scale mirror displacements can modulate the emission spectra of trivalent europium ions within their excited state lifetime. The dynamic LDOS modulation presented here can be readily extended to faster optical modulation schemes and applied to other long-lived emitters to control the direction, polarization, and spectrum of spontaneous emission at sublifetime scales.