What happens when you connect your new iPhone to your old iTunes?

I have two separate Apple accounts now. Will I lose the songs that I've downloaded from iTunes on my new iPhone? Will the old iTunes songs be uploaded onto my iPhone? Is this a wise decision? Any additional advice and recommendations are welcomed.|||If you plug your iPhone into the computer, iTunes should pop up and give you option to sync the phone with everything currently on your itunes. and then you shouldn't lose anything you already had.