We study the cosmological constant (Lambda) in the standard Lambda cold dark matter model by introducing the graduated dark energy (gDE) characterized by a minimal dynamical deviation from the null inertial mass density of the Lambda in the form rho(inert) proportional to rho(lambda) < 0 with lambda < 1 being a ratio of two odd integers, for which its energy density rho dynamically takes negative values in the finite past. For large negative values of lambda, it creates a phenomenological model described by a smooth function that approximately describes the Lambda spontaneously switching sign in the late Universe to become positive today. We confront the model with the latest combined observational datasets of Planck + baryon acoustic oscillations + supernova + H. It is striking that the data predict bimodal posterior probability distributions for the parameters of the model along with large negative lambda values; the new maximum significantly excludes the Lambda, and the old maximum contains the Lambda. The improvement in the goodness of fit for the Lambda reaches highly significant levels, Delta chi(2)(min) = 6.4, for the new maxima, while it remains at insignificant levels, Delta chi(2)(min) less than or similar to 0.02, for the old maxima. We show that, in contrast to the old maxima, which do not distinguish from the Lambda, the new maxima agree with the model-independent H-0 measurements, high-precision Ly-alpha data, and model-independent Omh(2) diagnostic estimates. Our results provide strong hints of a spontaneous sign switch in the cosmological constant and lead us to conjecture that the Universe has transitioned from anti-dc Sitter vacua to de Sitter vacua, at a redshift z approximate to 2.32, and triggered the late-time acceleration, and suggests looking for such mechanisms in string theory constructions.