Learn English – the opposite of “in name only”

I think "We don't call it that, but in practice it really is" is a pretty good definition for de facto.

actual; especially: being such in effect though not formally recognized

Words that have opposite meanings depending on context are called contranyms; Janus words, from the Roman god of doors between beginnings and ends. It's not exactly what you're looking for, but the term might get you on the right path.

But I'd guess most examples include neologisms — new colloquialisms we may use but might not be widely "accepted" yet, like "sick" meaning an awesome new DLC for a staid game, where the word ("sick") also retains it's everyday meaning of "not well". The neologism ("sick" with a positive connotation) is then combined with an antonym for the original word's meaning (antonym of "sick", like "healthy"), to produce the effect the OP is asking for: neologism[word] + antonym[word].