My experience with BAH is that if you make a voluntary change they always say something like "we would normally charge X" and then always follow-it up with something like "but because we love you so much we won't"....
(I may be listening to the world through rose-tinted hearing-aids !)

They would be well within their rights to start charging, though.
Who knows, maybe they will in the near future.
But I wouldn't say that the above experiences are new or unusual.