  The conservation of wave action in moving plasmas has been well-known for
over half a century. However, wave action is not conserved when multiple wave
modes propagate and coexist close to degeneration condition (Sound speed equals
Alfv\'en speed, i.e. plasma $\beta \sim 1$). Here we show that the violation of
conservation is due to wave mode conversion, and that the total wave action
summed over interacting modes is still conserved. Though the result is general,
we focus on MHD waves and identify three distinctive mode conversion
mechanisms, i.e. degeneracy, linear mode conversion, and resonance, and provide
an intuitive physical picture for the mode conversion processes. We use 1D MHD
simulations with the Expanding Box Model to simulate the nonlinear evolution of
monochromatic MHD waves in the expanding solar wind. Simulation results
validate the theory; total wave action therefore remains an interesting
diagnostic for studies of waves and turbulence in the solar wind.
