What makes Pratchett's later Discworld novels different from the early ones is the same thing that makes any late work different: the writer knew they were mortal. The early Discworld is playful with death as a concept. The later Discworld — Guards! Guards! through to Raising Steam — treats it with the respect of someone who has thought about it professionally for thirty years and personally for the last decade.

Night Watch is the pivot point. It's the first Discworld novel that is fully tragic in its emotional register while still being funny. Vimes goes back in time, meets the younger version of the mentor who shaped him, and cannot prevent that mentor's death. The book is about what people sacrifice to become who they are, and the cost is not abstracted or softened.

I think Pratchett was one of the great moralists in English literature and the comedy was the delivery mechanism, not the content.