There's another form of this: the shifting of the dividing line between "microevolution" and "macroevolution". As scientists discover more and more extreme examples of genetic or morphological change within populations, creationists change the definition of "microevolution". In recent years it's gone from covering only changes in the relative frequency of brown vs. white moths, to covering the separation of closely related species. No matter how dramatically evolution progresses in the future, they'll be able to say, "oh, that's just microevolution."