Flexible behavior requires dynamically updating which items in working memory (WM) are prioritized for upcoming action. Although shifting priority between items in WM is known to incur a temporary behavioral cost, the neural mechanisms that govern the magnitude and fidelity of these internal priority shifts remain poorly understood. Here, we recorded human electroencephalography (EEG) while participants performed a multi-item orientation working memory task requiring cue-guided shifts of priority between memorized items. Behaviorally, switching priority between WM items incurred a performance cost that scaled with the angular distance (magnitude) of the priority shift. Neurally, whereas frontal theta (4–8 Hz) power increased generally on priority-switch trials regardless of shift distance, central-parietal beta (15–25 Hz) desynchronization scaled directly with the magnitude of the priority shift and predicted the neural decoding fidelity of the newly prioritized representation. These findings identify human beta oscillations as a key signature tracking both the extent and precision of internal priority reconfigurations in working memory.