Isothermal aging of the disordered dielectric crystal KTa1 - xNbxO3 (x = 0.027), the symmetry of which is broken by the biasing electric field E, is studied by means of AC dielectric-constant measurements. The asymmetry of the crystal induced by the field is put in evidence by the response of the dielectric constant to the small field change δE. This response contains an anisotropic contribution proportional to δE and an isotropic positive contribution proportional to (δE)2. The former slowly decreases whereas the latter increases as time elapses. Our results are well interpreted in the framework of a model which attributes the time-dependent part of the dielectric constant to the evolution of the polarization domain walls: the slow decrease (aging) of the linear contribution is related to the decrease of the total wall area, which is a consequence of the domain growth, while the fast increase (rejuvenation) of the quadratic contribution is the signature of wall reconformations.