Hi,
Welcome in Microsoft community.
I am afraid to say that Microsoft planner currently does not support moving and/or copying tasks in bulk between different plans under the same group. But you can copy or move the tasks one by one from one Plan to the other.
For your reference, see: Copy and move Planner tasks (microsoft.com).
If you need to copy more than just a task, it currently only supports Copy a plan .
You can provide feedback in Microsoft planner to suggest your idea. To do so, go to Settings > Send Planner feedback.
Have a good day.
Regards,
Christophe