We tested the idea that local changes in action demands (e.g., due to an invalid cue or trial-by-trial) result in frugal modifications of existing action plans via action-plan-modification operations. We implemented an experimental procedure making use of a cue that indicates the action requirements for an upcoming signal with a certain degree of reliability. Crucially, incongruent cue-stimulus pairs either require action-plan modification or "resetting" the prepared action plan and reselecting a new response from scratch. We systematically varied the proportion of valid cues over four experiments. There were four response conditions: left button press, right button press, dual button presses, no action. Results support the concept of action-plan modification rather than reset-reselect: switching between a left and a right response was faster and less error-prone than any other type of switch, both between trials and between cue and signal. Thus, it appears that given two responses that can be conceived of as polar opposites (within the same single-action category), there is an action-plan-modification operation ("invert") that transforms one into the other at a comparatively low cost.