This paper explores the role of R&D worker mobility on innovation performance. As one main novelty, we employ churning as a measure for worker mobility. Churning depicts the number of workers which are replaced by new ones. It is a very informative indicator since a firm may be exposed to simultaneous leave and inflow of R&D workers even if the size of R&D employment remains unchanged. Hence, we can separate the effect of replacement from net change in R&D workforce. Our results from estimating various knowledge production functions suggest an inverse u-shaped relationship. The exchange of R&D personnel fosters innovation through inter-firm knowledge spillovers and improved job-match quality up to certain threshold. The point when costs of churning exceed the benefits is reached faster if the R&D knowledge is non-duplicative.