Casting v. Canon
rand al'thor shows that Hermione could alter her complexion and there are dozens of spells that can transform wizards into other shapes, colors, animals, and people. Victor Krum turns into half a shark. Barty Crouch Jr. spends a year as Mad-Eye Moody. Peter Pettigrew lives for decades as a rat. The magic surely exists for "Emma Watson" to spend the duration of "The Cursed Child" looking like "Noma Dumezweni" but it isn't productive to try to incorporate recasting into canon.
Lavender Brown was played by two different black actors and one white actor.
If changing your appearance to another race requires "stronger," "more experienced" magic as the question suggests, why is a not-very-clever, 12-year-old Lavender so good at it?
Dumbledore was played by two different white actors.
We aren't having a discussion about when and why Dumbledore changed his face, or if it's possible he's a Time Lord. This conversation only arises when it's about race.
We're used to holding separate canons in our heads across media (books to film) but here's something that happened within the movie universe that is not getting the same attention. Maybe because the character was white by the time she got a speaking role?
As a person who works in professional theater, I can tell you that plays are different than books and films in that they are written to be performed many times by different casts on different sets and with different directorial visions. Generally in theater, "canon" is the text of the play, not the choices of the production.
JK Rowling (who also contributed to the films) collaborated on the script for the play with Jack Thorne (who is listed as the playwright, based on a "story by JK Rowling") and Rowling has tweeted that the "story" of Cursed Child is canon.
However involved and supportive Rowling is, the specific production will be created by many artists, much like the films were. Rowling has been enthusiastic and supportive of many official, derivative works by other artists (including the films, book artwork, etc.) without suggesting that they alter her canon, and that is a good way to understand the gap between the fact of the story of The Cursed Child and the details of any given production of The Cursed Child.
Another theater example: Casting people of color in Hamilton doesn't change the fact that the founding fathers were white, but it does do something more nuanced and important.
This is mostly just to say that the tunnel vision of "canon" can be unhelpful to understanding a work of fiction, especially when it works to exclude the same non-white, non-cis, non-male people that science fiction and fantasy has a problematic history of excluding.