We examine with particle-in-cell (PIC) simulations how a parallel shock in pair plasma reacts to upstream waves, which are driven by escaping downstream particles. Initially, the shock is sustained in the two-dimensional simulation by a magnetic filamentation (beam-Weibel) instability. Escaping particles drive an electrostatic beam instability upstream. Modifications of the upstream plasma by these waves hardly affect the shock. In time, a decreasing density and increasing temperature of the escaping particles quench the beam instability. A larger thermal energy along than perpendicular to the magnetic field destabilizes the pair-Alfven mode. In the rest frame of the upstream plasma, the group velocity of the growing pair-Alfven waves is below that of the shock and the latter catches up with the waves. Accumulating pair-Alfven waves gradually change the shock in the two-dimensional simulation from a Weibel-type shock into an Alfvenic shock with a Mach number that is about 6 for our initial conditions.
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