Go to the issue view and use the "more" menu (...) to select "convert to sub-task". It will ask you what type you want to change it to, and what parent issue you want to move it under.
If you don't have that option in the menu, then your project "issue type scheme" does not have any sub-task types in it and/or "sub tasks" are disabled in global admin.
I don't use Core, but I'm sure it's the same as the other Jiras. Click on the three dots (or More in older versions) at the top right hand side and there should be a menu item Convert to sub-task. it shouldn't make a difference which state it's currently in. You would only see this menu item if you have the correct permissions.
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