The ability to discard a prepared action plan in favor of an alternative action is critical when facing sudden environmental changes. We tested whether the functional contribution of left supramarginal gyrus (SMG) during action reprogramming depends on the functional integrity of left dorsal premotor cortex (PMd). Adopting a dual-site repetitive transcranial magnetic stimulation (rTMS) strategy, we first transiently disrupted PMd with "off-line" 1 Hz rTMS and then applied focal "on-line" rTMS to SMG while human subjects performed a spatially precued reaction time (RT) task. Effective on-line rTMS of SMG but not sham rTMS of SMG increased errors when subjects had to reprogram their action in response to an invalid precue regardless of the type of preceding off-line rTMS. This suggests that left SMG primarily contributes to the on-line updating of actions by suppressing invalidly prepared responses. On-line rTMS of SMG additionally increased RTs for correct responses in invalidly precued trials, but only after off-line rTMS of PMd. We infer that off-line rTMS caused an additional dysfunction of PMd, which increased the functional relevance of SMG for rapid activation of the correct response, and sensitized SMG to the disruptive effects of on-line rTMS. These results not only provide causal evidence that left PMd and SMG jointly contribute to action reprogramming, but also that the respective functional weight of these areas can be rapidly redistributed. This mechanism might constitute a generic feature of functional networks that allows for rapid functional compensation in response to focal dysfunctions.