re-defining tasks as subtasks in jira

[Tim Williams]

How do we re-define existing tasks as "Sub-Tasks" in JIRA?  I thought
that creating an "is part of" relationship would make it a subtask but
it apparently doesn't.  Do you have to create a new sub-tasks and
merge it or something?  The example I'm working with is FOR-200 (the
major task) and FOR-573 (the sub-task of FOR-200).
Thanks,
--tim