What you need to do is that, once you detect a new object, call the global planner to get the global planner （make plan) according to current position and send the plan to the local planner (set_plan). That could errase the last goal (last path) and track the new goal(new path). move_base provides an action to complete navigation task. When you add a new action, the previouse one would be preemted and the robot would stop the old task. I used to achive this by add a update path service in move_base. By doning this , I can change the tracking path during navigation task runtime. It worked but I don't like this method. I suggest the move_base_flex stack (upgration version of move_base) to implement complex strategies by smacc or pt_tree.