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!
In the past, Portfolio for Jira required a high degree of detail–foresight that was unrealistic for many businesses to have–in order to produce a reliable long-term roadmap. We're tur...
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