I'm convinced that the spec change in the iPad Air 2 from bt 4.0 to 4.2 is not a hardware change.
1) the model numbers didn't get a revision change.
2) the iPhone 6 also shows change from bt 4.0 to 4.2. Really don't seem them doing a hw spec change on it at this point.
My guess is that anything with wireless ac was hw capable for bt 4.2 and was a candidate for sw upgrade.