I'm sure Sir Terry's estate -- and in particular his daughter, who I think is his literary executor -- will have thought carefully about all the changes before approving them.
Rhi Pratchett is no longer involved in the project and has no say on casting, or any other matters regarding the direction of the show.
I mean, Anna Chancellor is playing Vetinari, Ruth Madely is playing Dibbler, Bianca Simone Mannie is playing a completely new character, Wonse, ‘a wizard hopeful in waiting that is frequently underestimated' when wizards in Discworld are all male, and Lara Rossi looks far too young and slim to be Lady Sybil, so clearly they've taken a considered decision to depart from the books quite dramatically, at least when it comes to the characters' physical appearance.
Most of the changes don't bother me, Anne Chancellor as Vetinari for instance, actually excites me. Stepping into Charles Dance's shoes is a big ask, but I think she'll be more than capable of making the character her own. Lara Rossi's casting annoys me a little, in that it
appears to be falling into the "young and pretty love interest of the middle aged dude" trap that so much tv and film falls into. Though the fact the role's being used to up the PoC diversity tempers my annoyance somewhat.
Wonse is a bit of a weird case. The female wizard thing I can see, beyond
Equal Rites Sir Terry rarely used the boy's club nature of Discworld wizards as anything more than the odd joke about their (lack of a) sex life. The name however, is attached to a character that was fairly central to one of the earlier Watch novels. If this Wonse is somehow going to fill a role similar to that of Lupine Wonse, all's well and good. If however, she's an entirely new character then I'm a little less copacetic. That would indicate to me at least, a lack of care for the Discworld property on the part of the showrunners. Mining earlier works for names, without regard for what names are being used and in what context, is not something you do if you care about the integrity of a setting.
In any case, I'm not premptively giving up on things. For all I know, my worries will be for naught and things will be fine. Even if they do take transformative liberties, it could still work. I'm mean look at
Lucifer. That show bears almost no relationship to it's source material and it's still smart and funny and very, very watchable. There's no reason yet that
The Watch can't be too.