A:
I am in this same boat. I have an unlimited data plan for my non-smartphone, but AT&T detects the M8 and adds a data plan.
I need to constantly switch SIMs into the non-smartphone and call AT&T to switch back to data unlimited.
If you have a true SMARTPHONE unlimited plan, they will grandfather you.
If the unlimited plan is for a non-smartphone, you're gambling that they don't switch you.