Sales Rep in store completely screws up my account and now I'm forced to suffer the repercussions!!!

Back in November 2016 I went into an At&t store to purchase an iphone 7plus. Keep in mind that my sister is also on my account and has an iphone 6. I was told by a customer service agent that I was not eligible for an upgrade, so in order for me to upgrade I needed to pay $250. I said, Cool. Swiped my card. Paid the balance of my bill, reviewed and signed all the documents, and put in the order for the phone to be shipped to my house. I should have known when the phone arrived and my sister's number was on the paper instead of mine, inside the box with my phone that something was very very wrong. Fast forward to July of this year and I'm in vegas, just waking up in the morning and I see $573 added to my current bill!!! I immediately call and they inform me, that a line was canceled and that was the remaining balance. Now this is where it gets tricky. My sister and I got iphone 6's March of 2015, hence why November of 2016 I wasnt eligible for an upgrade, as previously mentioned. My sister ported her number to another carrier. But... She only did that because she was told in February of 2016 when we made a store visit that by August of 2017 she would only owe around 50-100. You're probably confused by now. But let me clear it up. When I went and got the "upgrade" in Nov 2016, the sales rep failed to inform me that he was actually performing a "cross-upgrade" which basically means that he upgraded my sisters line instead of mine. Because of this, in the system unfortunately, it looks as if the iphone 6 started a new installment and my iphone 7plus just continued with the regular bill. Basically, he swapped the installment plans. So when my sister ported her number out, the balance from my new iphone7 plus, appeared on the bill and it says I only owe $50 for my phone to be paid off. I have been back and forth to that same store 3 times, I've spoken with 5 different AT&T reps over the phone and no one has helped. All I want is for them to switch it back and every person I speak with acts as if they dont have slightest clue of how to do their job. My bill is $983 right now and I absolutely refuse to pay that when it was At&t who messed up my whole account and refuses to fix it. My service will be cut off soon because no one wants to work with me about anything and I'm honestly just very frustrated and fed up. I've been with At&T for almost 5 years now and its very inconvenient and unprofessional to not want to do whatever you can to rectify the problem that would never have been as issue if I was fully informed of what kind of "upgrade" he was performing as well as what it meant etc. Thats all just to say that as soon as I can, I will definitely be leaving and probably never coming back. I've lost all respect. Period.