Existing customer with Insider Code. How can I take advantage of this? Currently on ONE plan

I have been on the ONE plan for a long time. 6 lines total (1 free line). My buddy at TMO just gave me an Insider Code. He said I would have to port a line over to another carrier and then port back in with a new account. Just want to confirm this will work. Will I lose my free line when I port all lines over to the new account? TIA