Going back on a project is a step backwards but it’s a necessary action to commit. Trying to continue a project without going back and correcting prior mistake will lead to a larger issue further down the road. Also, if the schedule change is coming from higher management it would be hopefully understood if the timeline of the project changes. If they are not it will fall on the PM to fix the schedule to be more rigorous and allow the staff to pick up the extra weight from the project change.