There are a number of possibilities. One, indeed, is oversight. Another is Harley, possibly on one of her less rational phases, using whatever name comes to mind, or strikes her fancy. Lastly, it's entirely possible that they're an entirely different set of hyenas in that instance. Hyena lifespans are only about 12 years, and that's not including the wear and tear from fighting Batman, the time Harley used them to test deadly diseases, the tendency for minions to die around Joker... there's a decent chance that we're on our third or fourth set of Bud and Lou.
For what it is worth, I haven't found any references to them being named Giggles and Chuckles outside of a few pieces of fanfiction (where the author generally said they made up the names themselves) and in Birds of Prey, the (single) hyena was named Bruce, after Bruce Wayne.