Google added new Tasks features to Gmail that allows you to easily move a task, and all of its sub-tasks to another list. To do this, simply click on the arrow or use Shift-Enter to the right of the task. At the bottom of this screen is a drop-down with all of your task lists. Simply select another list, and leave the screen by clicking "Back to list." Your task will magically migrate to the other list.
Another new feature should help you can split a single task into two tasks or merge two existing tasks. If you've ever used a word processor, you already know how to do this. Simply use Enter and Backspace as you would normally.