For those, who is still on Jira 6.4 here is another workaround for massive convert from sub-task to standard issue type. Please, find attached file DYConvertSubIssueToIssue.zip. It is based on Matthias Hößl's solution, but much easier to use.
1. Get target issue type's ID. For example, from URL "Edit" against required issue type on "Issue Types" admin page.
2. Unzip, open & copy script text to buffer
3. Open Issue Navigator and enter JQL to find all sub-issues to convert (you may start from 1 issue by adding 'and issuekey = PRJ-111' at the end of JQL). Script will take jql string from URL bar (param ?jql=...).
4. Open developer tools -> Console, paste scripts' code. Edit target issue type ID in first lines.
5. Run script. It should show issue keys and Step OK four times each issue.
1. Target issue type shouldn't have additional requires fields. Existing subissues should have all required fields filled.
2. "Step is OK" mark actually means nothing, it just states, that Jira's returned HTTP code 200 on request. Check Issue Navigator after script finished. If an issue wasn't converted, try to convert it manually, may be one of required fields is empty.
3. Script expects, that base Url is configured correctly.
4. Jira creates unique wizard's ID for each issue. It can prevent immediate re-run of the script.