I'm on a AT&T plan with grandfathered unlimited data. I fulfilled my 2-yr contract a long time ago and have stayed on the same plan for years. I'm using an iPhone 3GS, which uses the old fat SIM cards. If I buy an unlocked T-mobile iPhone 6, what do I need to do to transfer my service from my 3GS to the new phone? Can I simply buy a http://www.amazon.com/AT-Nano-SIM-Card-iPhone/dp/B009DRLCXK/ref=cm_cr_pr_product_top plop it into my phone, and call AT&T to transfer my service to the new sim/phone? Is it that easy? And if I do this, will they muck with my grandfathered unlimited data plan, which I don't want to lose?