There is some EC in the already released (in Japan) Dragon Quest XI:
At one point in the game you return to a town named Grotta to find that there's a casino set up and humans and demons are living together. You go through the casino and earn your way up to the VIP lounge only to find out that a demon named Boogie has used his powers to transform the people in the town into demons.
Once you get up to the VIP lounge, you are faced with a former party member, Martina, who has been turned into a demon and now works for Boogie. Here's a video of her introduction: (and boss fight)
https://youtu.be/20XXFe1nohU?t=39m27s
There doesn't seem to be an process, but after you beat Boogie and Martina re-joins your party, she does end up with the ability to transform back into a demon in battle. This appears to be just an in-battle power up effect and doesn't change anything with her character or anything.
There's already at least one doujin that 'expands' on how Martina was turned into a demon:
https://e-hentai.org/g/1156258/8741d6d150/