Adiabatic three-wave mixing processes enable broadband, efficient, and robust frequency conversion by slowly varying the phase mismatch between the interacting waves along the interaction region. Up until now, this method was mainly used in the case in which one of the waves was undepleted. Here we experimentally study fully nonlinear adiabatic processes by implementation in type I and type II second-harmonic generation processes, where the undepleted pump approximation does not hold. Using quasi-phase-matched interaction in chirped gratings, we obtain conversion efficiency approaching 60% and 80%, with corresponding wide thermal acceptance bandwidths of >100°C and 30°C, respectively. The transition between the depleted and undepleted pump regimes is also studied by varying the input polarization angle in the type II process; thus we also test current theory with arbitrary initial conditions. The results are in excellent agreement with analytic predictions for the fully nonlinear adiabatic process.