[inimino]

Bayesian analysis means you set up at least two models and evaluate the likelihood of the observed data under each. In this case rather than assume a fixed probability of good reviews, you might model a sudden switch (for example if an account was taken over or sold) or a gradual decline (for example if a manufacturer gradually lowers quality). Then you proceed as usual from that point. (I.e. find likelihoods and assign priors for each hypothesis/model, and use Bayes' rule.)