Any speculation? Beyond the fact that everyone's appearance has changed and Mark Lenard had passed. I'd love to hear theories!
The characters have not changed appearance. We simply accept the fact that they are being played by different actors.
I'm not sure what that sentence means. The characters are supposed to look the same as they always did, even if we the audience can see different actors playing them. So to the characters, Sarek looks just like he does in the Prime universe.