I have called Verizon three separate times and all three reps told me the same thing. If you are doing a alternate upgrade (a upgrade from another line already on your account that is eligible) what you need to do is before turning the phone on call them and they will activate it right to your number with no issues on keeping your unlimited data. I asked this question several times about the unlimited data each call they all said no problem this is a question asked 25 times a day.
I'm going to try to verify this as well. The rep I spoke to on the phone told me that I would need to first activate it on the eligible line and then go to the Verizon store so that they could transfer it to my line and give me a new sim card. As such, I've been trying all my local verizon stores to see if they can issue me a new sim card now so that I don't have to go to the Verizon store with my iPhone 5 on launch day morning so that it'll work with my line.
Not saying that you're wrong - I hope you're right. I just hadn't heard of that option. I'll let you know what I find out.