[notahacker]

Yep. The most jarring changes of style, worldbuilding, themes and characters are in the first four books (Pratchett once responded to a question about whether "the Patrician" in The Colour of Magic was the same man as Vetinari by saying yes, but he wasn't the same writer). The non Alzheimer's explanation for changes in vocabulary around the turn of the century is that he'd started writing Young Adult Discworld books (he'd written for younger audiences before, but in very different worlds) and for better or worse some of the changes bled through to his other works. Of course, that's also compatible with him having less capability to write complex books, but then I'm not really convinced Thud is less complex than The Colour of Magic...