We use JIRA at work for bug tracking. There is no way currently to convert "Tasks" in bulk to "Sub-Tasks." I needed to convert about 30 tickets, so I wrote some JavaScript I could just copy/paste into the browser console on each page load. Saving it here in case this happens again.

convertToSubTask.js

function convertToSubTask (taskID) {
    "use strict";
    // Returns are required otherwise the rest of the if statements will attempt to execute.
    // A switch statement would be easier to read and maintain.
    // taskID must be a string.

    if (jQuery("#issue-to-subtask").length > 0) {
        window.open(jQuery("#issue-to-subtask").attr("href"), "_self");
        return true;
    }

    if (/ConvertIssue\.jspa/i.test(window.location.href)) {
        jQuery("#parentIssueKey").attr("value",taskID);
        jQuery("#next_submit").click();
        return true;
    }

    if (/ConvertIssueSetIssueType\.jspa/i.test(window.location.href)) {
        jQuery("#next_submit").click();
        return true;
    }

    if (/ConvertIssueUpdateFields\.jspa/i.test(window.location.href)) {
        jQuery("#finish_submit").click();
        return true;
    }

    if (jQuery("#issue-to-subtask").length === 0) {
        window.open('', '_self', '');
        window.close();
        return true;
    }
}

jQuery(convertToSubTask("MKT-8603"));