  We consider a coupled mode system where the effective propagation constants
of localized modes are amenable to modulation. Starting from an unmodulated
system where power transfer is heavily suppressed, we demonstrate that small,
periodic modulation of the propagation constants enhance power transfer using a
slowly varying envelope approximation for the field mode amplitudes. We
calculate an approximate modulation frequency enabling complete transfer
between otherwise negligibly coupled elements. The ability to control these
modulations by electrical or thermal effects allows for reconfigurable
multiport switching. We use an array of coupled silica waveguides and the
thermo-optic effect to test our predictions in the telecom C-band. However,
this requires a refractive index modulation with period of the order
$~10^{-3}~\mathrm{m}$ and yields total power transfer with a propagation
distance of the order of $10^{-1}~\mathrm{m}$, which might make it unattractive
for integrated photonic applications. Nevertheless, our results are valid for
devices described by an equivalent coupled mode matrix for space or time
propagation; for example, arrays of microring or terahertz resonators,
microwave cavities, radio frequency antennas, or RLC circuits.
