I'm having trouble getting a solid answer on this from the web. For instance, this page offers contradictory answers. Some saying it will be fine, some saying that AT&T does not grandfather when upgrading to a 6.

I've had an AT&T plan with unlimited data for years. Each time I get a new model of iPhone, I have been grandfathered into the unlimited data plan each time. For instance, my last upgrade was to an iPhone 4s, and it still has the unlimited data plan.

Normally I would expect this not to change, but the 6s will be different from my current 4s in one important aspect: The 6s will use LTE. I would think that might change the landscape slightly? Maybe I would need a different plan if I want to use LTE, and thus lose my grandfathering? Not sure.

Does anyone know?