ATT Next

As has already been said, if the phone is fully paid for, you do not need to trade it in.  You can choose to trade it in or keep it if you want to replace it before it is fully paid for.  If you do choose to keep it, and it isn't fully paid for, you will be required to pay off the balance remaining balance on it,  and if you choose to upgrade before you reach the "upgrade point" and you want to use the "trade in option, then you will still need to make a one time payment equal to the amount you owe to get to the upgrade time.

SO lets say the phone is 800 , it required you to pay ~ 600 to be eligible for upgrade ( that 200 is waived since we're trading in ) ; so then we will start a NEW installment plan with let say another smart phone 800 again. Do we get any discount on it at all? or no . because if no, then it's like that 600 dollars just went to waste ( or count as a loan ) for using the phone  ( which is a lot ) ----

@tdpn329 Nope, no discounts. You are right, exercising the trade in option can be very expensive. Your phone is likely worth more paying it off and selling yourself.

>"Your phone is likely worth more paying it off and selling yourself."

Not always.  It depends.  For instance, I'm on the "New phone every year" Next plan and have paid off half of my iPhone 7 Plus ($435.00), leaving a balance of $434.99.  But I want to switch back to the Samsung Note 8.  So I can trade in my iPhone7 Plus (in perfect condition) to eliminate that $434.99 that is still owed on that phone, and then start a new installment plan for the Samsung Note 8.  Or I can get $310.00 for the iPhone 7 Plus through Gazelle.com.

Trade-in value - $434.99

Sell vale - $310.00.

Obviously, I'm trading in!  

@Lloyd S In your case that is likely the right choice but I would not consider Gazelle to be "selling it yourself" nor would I take the Gazelle price as the definitive sell value. I see prices as high as $455 for a 128GB iphone 7s plus on ebay.