Dont take everything ONE At&t rep tells you for the truth. I once had a bunch of them telling me I COULD NOT use the WiFi on my Blackberry withOUT data??!!
Anyway, just upgrading to a 3G iPhone on one or two of the lines and singing "a new 2-year contract" will most likely NOT change your rate plan, those rate plans are most likely for NEW customers. It seems the same whenever your upgrade with At&t, your contract is extended, but your RATE plan remains the same. That's why unless you actually change your rate plan you can continue to be grandfathered from year to year on some realllllly old rate plan. The "new" 2-year contract will probably just extend your contract lenght, not actually change your rate plan.
This is what happened to me when upgrading in the past and I dont plan it to be different. I spoke to a few At&t reps and they all told me my rate plan would not change. I have the Nation 550 Family plan with my iPhone as the SECOND line (non-iphone has primary) and when I add the 3G to my 2ndary line I do not plan that changing the rate plan on the primary phone.
To run a sanity check, I called one of our local stores (first call was to Business Care 800 #). The rep at the store says they are trying to get clarification on the "requirement" for existing Family Talk plan customers to "upgrade" to new Family Talk plan when choosing the iPhone 3G. I'm supposed to call back tomorrow. This should be interesting.