Since you asked, heres my take:
I think changing Triss, Fringilla, Istredd and a few other minor characters does not need to be, but does not disturb the world itself that much as long as its done right. The Continent definitly has space where you could reinterpret peoples ethnicity it seems like in their take zerrekanians and maybe some others are black wich could make sense geographically if you move it a bit more to the south into korath desert, and since the story never goes there why not. Then having these people migrate into Nilfgaard and the Nothern Kingdoms isnt that unlikely either, they are not that far away in the east, plus through magic the witcher world is far more connected and younger than the middle ages so information spreads further leading to more interest in migrating somewhere, plus the humans arrived 2000 years ago, not enough time to develope different races but since they exist in books and tv show they had to exist when they arrived and some of them staying is possible.
Additionally:
I know there are people who say that the showrunners statement that there will be no new interhuman racism changes the story because the people in the witcher are racist towards everything different, but i dont think that has to be the case. The nazis brought comepletly different people on the same mindset, by giving them a common enemy. The fact that in the witcher humans are only on the continent for 2000 years and didnt really have time to spread or develope different races on the continent, hints that they arrived with different races (books and tv show) so since they always new about other races and had that common enemy would result in an entirely different way of racism. After that common enemy is gone interhuman racism would rise but it does not have to be there at the point the events of the show play out. It should be adressed that some are different but they dont need to be discriminated.
But the question to me is not wether these changes are explainable, because to me they are, in the witcher they are atleast possible, but wether they have a reason and are done right. I think you shouldnt always believe a companies explanation for a decision, companies rarely follow their own moral code, thats marketing, they care about money, no this decision was made with the believe that the witcher show now has a higher chance of success and if these changes guaranty that im getting the full story i can accept a lot and this really does not bother me at all, its their adaption after all, we cant imagine them changing the book and slowly insert all their changes into them and wonder why they dont fit, we see the full new picture and then judge if its good, in a new take i could totally buy other ethnicities existing.
Me not caring at all could also have to do with the fact that while im a really big witcher fan who read the books and played the games multiple times, the characters that changed are not my favourites. If lets say yennefer or ciris appearance changed drastically, maybe i would have a different opinion, to me the changes in the end just dont matter that much, because fringilla is explainable, just have geralt morn yens name during sex and in sleep without here looking like here, and triss and istredd and the other not that prominent characters have not much speaking against it, i cant understand why fans of these characters are upset but i am not i think their character can exist while looking different. (I do really feel for the game triss fans, they are gonna hate the TV show anyway since CDPR changed her so much)
I believe if its done right and the feeling of the universe is mostly unchanged the main audience who does not know the witcher wont care and that it wont be a big problem for the core fanbase, just like the games they will have to change quiet a few things to makes their adaption work and these wont be the most controversial, but if its done right no one will care like it was with every adaption ever(GOT LOTR Witcher games etc).
So in the end, these changes for me did not have to be, but are acceptable and explainable and im very happy we are getting a new witcher TV show that so far looks great to me, I am really looking foward to hear their soundtrack after its finished i hope theyll use it in their next trailer.