  We numerically investigate the imprints of gravitational radiation-reaction
driven changes to a black hole's mass and spin on the corresponding ringdown
waveform. We do so by comparing the dynamics of a perturbed black hole evolved
with the full (nonlinear) versus linearized Einstein equations. As expected, we
find that the quasinormal mode amplitudes extracted from nonlinear evolution
deviate from their linear counterparts at third order in initial perturbation
amplitude. For perturbations leading to a change in the black hole mass and
spin of $\sim 5\%$, which is reasonable for a remnant formed in an
astrophysical merger, we find that nonlinear distortions to the complex
amplitudes of some quasinormal modes can be as large as $\sim 50\%$ at the peak
of the waveform. Furthermore, the change in the mass and spin results in a
drift in the quasinormal mode frequencies, which for large amplitude
perturbations causes the nonlinear waveform to rapidly dephase with respect to
its linear counterpart. %These two nonlinear effects together create a large
distortion in both the amplitude and phase of the ringdown gravitational
waveform. Surprisingly, despite these nonlinear effects creating significant
deviations in the nonlinear waveform, we show that a linear quasinormal mode
model still performs quite well from close to the peak amplitude onwards.
