If you're looking to do this for one specific set of tasks/subtasks, you can do with so with a bulk action, or just using the CLONE option.
If you're looking to do this regularly, I would suggest writing an automation script to do it instead -- but that's a bit more complicated and might require explanation.
If you look in the "..." menu for any particular issue, you'll see "CLONE" as an option, which also will give you the option of cloning the subtasks:
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