Practicing retrieval can improve the updating or modification of existing knowledge. When students need to update their existing knowledge, performing retrieval practice on the first set of materials often strengthens learning of the next set. However, Davis and Chan (2015)reported that interpolated testing can sometimes impair new learning. Here, we examined whether frequently switching between retrieval of previously learned material and encoding of new material can disrupt learning of the new material. In the current experiment, participants either switched between restudying originally learned items and new learning or between retrieving originally learned items and new learning, and we varied the frequency with which task switching occurred. We found that interpolating retrieval, but not restudy, with new learning impaired new learning. These results are consistent with the idea that retrieval practice and encoding rely on different cognitive processes, and intermixing them can exert a cost.