To elucidate the time course and processes underlying pre-movement modification of planned actions, participants prepared to make an action at a time of their own choosing within a specified temporal window. In some conditions, participants prepared to make a single right index finger key press, whereas in others, they prepared to make a sequence of two key presses consisting of a right index finger key press followed by a right middle finger key press. On a proportion of trials, their internal preparation was interrupted by an auditory tone, in response to which they made either: the same action as they were intending, a different action requiring an additional effector (i.e. switch from preparing a single right index finger key press to executing a right index, middle finger sequence), or a different action requiring one less effector (i.e. switch from preparing a right index, middle finger sequence to executing a right index finger key press). For unmodified actions, switching from an internally generated to an externally triggered mode of response production produced a significant reaction time cost (RT cost) for both single and sequential actions, with the cost for single actions being significantly greater than that for sequential actions. Given that the RT cost did not increase as the complexity of the actions increased it is unlikely that the source of the cost is related to motor execution processes, and it is suggested that it may arise at a higher level cognitive stage of processing. In addition, reaction times to produce modified actions were significantly greater than those to produce unmodified actions. Finally, it took significantly longer to produce modified actions requiring one less effector than to produce modified actions requiring one more effector. We suggest that two time-consuming processes are involved in switching between internally generated and externally triggered actions that are modified or unmodified: a trigger switch cost when the same action has to be produced in response to an external trigger as opposed to an internal trigger, and a switch cost reflecting changes in the pattern of executed motor commands when modification is necessary. It is suggested that such processes may be mediated by regions of the frontal lobes.