https://www.polygon.com/2018/8/20/17760782/world-of-warcraft-battle-for-azeroth-faction-change

World of Warcraft: Battle for Azeroth is all about the faction war at the moment: Horde versus Alliance, red versus blue. But sometimes, being practical is more important than being sentimental. You may be an Alliance player, like I am, but having friends on Horde dictates that you have a Horde character ready to play as well. So while I left my main character on Alliance, I tested what would happen if you bought a faction change in Battle for Azeroth, switching another to the competing side.

After hitting level 120 with my third character, an Alliance Shaman, I decided that I would rather have this character on the Horde side, so that I could heal dungeons with some pals. I then purchased a faction change out of both necessity and curiosity. At the time of faction change, I had not only reached the max level, but I’d also completed the storylines of each Alliance zone.