High-density communication through optical fiber is made possible by wavelength-division multiplexing (WDM), which is the simultaneous transmission of many discrete signals at different optical frequencies. Vast quantities of data may be transmitted without interference using this scheme but flexible routing of these signals requires an electronic middle step, which carries a cost in latency. We present a technique for frequency conversion across the entire WDM spectrum with a single device, which removes this latency cost. Using an optical waveguide in lithium niobate and two infrared pump beams, we show how to maximize the conversion efficiency between arbitrary frequencies by analyzing the role of dispersion in cascaded nonlinear processes. The technique is presented generally and may be applied to any suitable nonlinear material or platform and to classical or quantum optical signals.