Custom service tasks deadlines date change

For tasks linked to custom created services, so which are not linked to a Companies House or HMRC official deadlines, it would be very useful to be able to change the deadline date of the task. As the system works now, the 'next due' date can only be changed from within the client record. If one does that, the system creates a new task and two tasks appear for the same service for the client, one of which is marked 'To delete'. Unfortunately, if you have workflows linked to the existing task, users will loose all their subtasks progress if that task is deleted. So if the deadline date could be changed for such tasks, this would mean that progress on existing tasks would not be lost