It's currently not possible to do that!
As you can see in the following link, JRA-13245, there's already a Suggestion made about this matter.
You can vote on this issue to enforce your desire for this feature.
The only known way to do this is to "Convert Issue to Sub-task", but this action cannot be done in a bulk change.
Hope this helps!
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
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