You port your current number to google voice. Then get a new number from att on a month to month Contract (if you're past your 2 year agreement). Porting your number cancels your att service. So if you're still under contract you'll har to pay an ETF. Of you have unlimited data, you likely won't be able to sign up for it again if you choose to stay with AT&T.
The best time to port your number to google voice is when you plan on switching carriers.