I currently have an iPhone 4 8GB on what's now called 'tab small' but at the time was just the tab. My current tab balance is -122.55. I want to upgrade to an iPhone 5S on the tab medium, so I have a few questions. 1 - How much would it cost to upgrade. I know that I have an outstanding balance so how would that effect the 300$ credit on tab medium? 2 - Can I keep my current plan. At the moment I have a 45$ plan with 1GB of data that was a special at the time I got it and no longer exists, so I was wondering if I can keep this when I upgrade. Thanks!