1. A method of processing a first classification model that classifies an input into one of a plurality of classes, wherein the first classification model was built using labeled training data having a first set of weights, wherein each item of labeled training data is labeled with one of the plurality of classes, the method comprising acts of: (A) obtaining unlabeled input for the first classification model; (B) using the unlabeled input to reweight the labeled training data to have a second set of weights that is different from the first set of weights by: building a similarity model that represents similarities between the unlabeled input and the labeled training data; and reweighting the labeled training data to have the second set of weights based on the similarities between the unlabeled input and the labeled training data indicated by the similarity model; and (C) using the labeled training data reweighted according to the second set of weights to: retrain the first classification model; and/or build a second classification model.