As I understand it you are currently doing training in the US on a J1 visa, and still have more than six months before you finish that training. You have received an Australian 407 visa for a different kind of training, which has to start within 6 months.
If you stop doing your training in the US then that will have some effects on you. Obviously you will not receive the qualification that you hoped to get from the US. You may be able to ask your US school if you can pause your training and finish it later. You have to ask the school, and it will be entirely up to them. If you just stop without arranging continuation then you probably forfeit any tuition payments you have made.
In any case when you stop the training in the US (whether or not you intend to continue it) you must tell your school who will tell US immigration. Your J1 will stop being valid, and you need to leave the US immediately. You will have to apply for another J1 if you want to continue your studies in the US. If you don't tell your school that would be immigration fraud and would seriously damage your chances of ever getting back into the US.
The alternative approach would be to contact whatever school you wish to study at in Australia and ask if you can delay the start of your studies there until after your US training has finished. You would then need to apply for another Australian visa. If you have been granted one already it is likely that you would be granted another for the same course.