You can upgrade to a refurbished phone after you are into your new contract in as little as 6 months...
I did this for 5yrs also, actually for 3yrs in a row AT&T let me purchase about 3 or 4 refurbished iphones in December EACH YEAR...I made about $1500 in December 3yrs in a row because of this
I was still able to do it last year before i switched to Verizon. This was on AT&T premier site though.
That's an early upgrade and costs $250 more than a full upgrade so around $450 plus taxes.
And you wont find any refurbished iphone 5 models for sale untill months after release.