We don't actually own a microwave, so we don't make microwave popcorn (we do, however, make it the old fashioned way on the stove because who doesn't like a monster bowl of popcorn every once in a while?) but for those of you who do, you might want to read this article about the fumes created from microwaving popcorn, which contains the chemical diacetyl.
As for me, this isn't actually a change, but a keep it up (i.e. I won't be making microwave popcorn).
~ Lynn