They haven't. What is in the books is in the books and will always be the same every time you read one. No one is rewriting the books here.
You can choose to watch The Watch with the opinion that it is Discworld in another trouser leg of time (or as I said on Twitter, In another trouser leg, in another pair of trousers, in a different shop on a different planet) or you can choose to watch it and sit there complaining at the screen over everything or you can not watch it at all.
It will never destroy the Discworld in your head unless YOU make it do that.