They've changed a few things -- mostly the flash-back motivations behind some of the victims, but that's acceptable because it doesn't change the story or structure, it just makes things a bit more relatable for modern day audiences. (In fact, having the flashbacks -- which I don't recall from earlier adaptations -- strikes me as a good idea, making the characters and past secrets more real. Oddly, they've also changed the name of the poem, which was the original name of the book, Ten Little Indians. I'm guessing they thought that a touch too politically incorrect, and made it instead "Ten Soldier Boys." (Actually, the original title of the real poem it's all based on was even far more politically incorrect.) But for the most part this adaptation by Sarah Phelps is very true to the novel. In fact, I'm curious if she's chosen to adapt the novel or Agatha Christie's play, which is what all subsequent films have done.
If you missed the first episode on Lifetime, they're repeating it several times. It's worth checking out. I hope that Part Two is done as well as the opening.