When I got the 5S and signed up for NEXT they grandfathered the data plan. (told them that was the only way I'd use NEXT) When I picked up the iphone 6 after a year they kept me on the same data plan, I paid only the taxes ($45) and my payment didn't change. So yes, they grandfathered the data plan into the NEXT program.
Are you sure you got grandfathered into NEXT WITH the unlimited data plan? Something really doesn't sound right there. As dfausty said, you can't do NEXT and keep your old unlimited data plan.