It is substantially different from changing a word in a terry Pratchett novel. Terry Prachett wrote none of that text. It would be absolute bonkers for Terry to claim ownership of text he didn’t write. Even if we pretend for a minute that the bot was asked to write in the style of Terry specifically.
But by prompting for 'in the style of' effectively you are mechanically rearranging everything he wrote without adding anything yourself. So not so different really, and I can see how lawyers for the plaintiff may make a convincing argument along those lines.
It’s a terrible argument and a terrible loop hole. It’s perfectly legal to hire someone to write in the copied style of Terry.
So even if your desired system were implemented to a T, you could hire someone to write a dozen or so examples of Terry writing. Probably just 30 or so pages of highly styled copied text, and then train your bot on this corpus to make “Not Terry” content. Boom. $100 on gig author and then for all practical purposes the Terry style is legally open source. Terry doesn’t even get the $100!
They've specifically proposed that there be a legal distinction drawn between "done by a human" and "done by an automated process."
Saying "a-HA! But my automated process can produce something that looks much like what your human would!" does not negate that; it merely makes it hard to tell the difference at a glance—which we already know to be the case.
