Words change and meanings change, all a natural part of linguistic development and evolution. Words that were utterly innocuous a hundred years ago can pick up huge amounts of cultural baggage, and words that could never have been said in polite company become ordinary. If you're in the middle of it, it's no problem.
When it comes to slang terms of the future you're writing, though, you've got to make choices. It's a key component of living language, one of the things that adds verisimilitude to a story, and you don't want to go the pulp sci-fi route, where characters said things like "space!" when from context it should have been more like "holy shit!" or "fuck!" - unless that, in and of itself, is a part of the setting itself, say a particularly puritan society that frowns on that sort of cursing. Culture isn't quite as fragile as it acted back in the 20th century.
The biggest issue, though, is intelligibility. The deftest slang in the verse won't be worth much if your readers can't grasp what you mean by it. For a real world example, take a look at rhyming slang - full of terms that, on the face of it, have little logical connection to each other. I mean, seriously; without the entire formation process laid out and lacking any prior experience with the slang, what kind of human would be able to pick up that "syrup" means "wig" from context alone?
So it is for the future, and my own philosophy has been to look for terms that can be adapted to slang, that fit with its rhythms, and that are not totally opaque in meaning. One that I particularly liked is "vuggy" - a mining term referring to empty, mineral-lined spaces in rocks. This lent itself well, I thought, to phrases like "vuggy bastard" or insults like "vughead." Plus, the V makes it seem all future-y.
Still, rhythm can lead you down potentially bad roads as well. This is one I'm struggling with in a current ongoing project:
"Modern anders didn't have quite as much raw physical strength as their neanderthal forebears, but she'd seen ander brawls that left elves and reggies with their skulls caved in."
The big question being, well, what's a reggie? Here, it refers to an ordinary, unmodified - hence, regular - human, in a setting where genetic engineering has been able to resurrect neanderthals ("anders") and create elves of a sort. I could hardly put "elves and humans" and keep it sensical within the logic of the story, since elves and anders are humans.
It's a question of opacity and of understanding - the answer will probably be different for everyone.