We had previously filed a petition for discretionary transfer. When we got the report back from the psychologist, he recommended keeping the juvenile in the juvenile system. Do I need to file either a new or amended petition alleging the offense, or is this a "lesser-included" type situation where we could proceed to adjudication under the petition for discretionary transfer?

Dismiss and refile especially since i'm assuming it's going to be going before the GJ.

Check the wording on your petition/motion. A petition for discretionary transfer should end with a plea to the court for transfer. Since that is not what you want - I think you should refile - whether or not you plan to take it to the Grand Jury (the wording on that one should be different, too).