AT&T will allow you to keep your old plans, even the unlimited plans. They can set up the installment billing on whatever plan you have. They strongly suggest that you change your plan due to the cost savings. Over the last 2 years of helping customers that have unlimited data, I have been able to show them the difference in pricing between their unlimited plan with a plan that fits their data use profile. The majority of them were able to save anywhere from $10-20 a month just by switching to a new plan and still be able to have enough data to cover their usage.
So, my recommendation for you and anyone else is, please really take a closer look at what the new plans are and calculate the monthly to the 2 year cost. Also, please remember that there are cell phone taxes that increases your bill. When you are calculating an estimated amount, your monthly installment billing amount is NEVER taxed, only the plan cost is taxed. What I mean by this is that if your 2GB plan is $30, your line cost is $25 and your IB cost is $31.21, then you will be taxed on $55 ($30+$25). After the taxed amount then the IB cost will be added. The reason for this is because you have already paid the taxes on the the phone when you picked it up. That is where the $60 of taxes come from.
I hope this clears up any confusion most have about IB. IF you have other questions, I will be more than happy to answer them.