The teaser trailer for Star Wars: The Last Jedi didn't actually offer up a ton of details on plot. It didn't show us a bunch of new characters to break down. That left fans to focus on the more minute details presented. And that left a lot of fans wondering, "Wait, what's up with Kylo Ren's scar?"
In The Force Awakens, Rey injures Ren with her lightsaber, leaving a big bloody mark that runs from his cheek, over his nose and into his forehead. In The Last Jedi trailer, Ren's scar is a much cleaner line over his eye. So what gives? The answer is simple: Director Rian Johnson thought the original scar looked bad, so they moved it.
This may drive the detail-oriented fans a bit crazy, but surely it's a better solution than having to apply a much larger, much more distracting bit of latex to Adam Driver's face every day (which is something J.J. Abrams may not have considered when gashing up Driver in The Force Awakens). And it's certainly not the first time that aesthetic changes have happened between Star Wars movies. Darth Vader's mask went from red eyes to blacker eyes. The Emperor underwent an entire face change.
Things like this happen in Star Wars. Embrace it. Just think of it as Rian Johnson's jedi mind trick. This is his equivalent of waving his hands and saying "This is the scar you're looking for."