Is there any option in PL/SQL Developer 12 to convert the queries written using the old-style joins (using "(+)") to new ANSI joins (using "LEFT OUTER JOIN")?

No, there is no option for this.

I would like to suggest this very useful implementation and can be obtained from the site www.sqlparser.com
Source code is also available (http://www.sqlparser.com/dlaction.p...e=SQL%20Formatter%20with%20source%20code).
Thanks for listening.

I have added this to the list of enhancement requests.