My current project just did this. A bad guy who I didn't think would be a bad guy just revealed himself. A bit early, but allows for major drama between the two throughout the rest of the book. I'm roughly 140 pages in.
Do you force yourself back to the plan you had in mind or do you brave it and let the characters take you on this new journey?
Right now I'm letting my characters take me on the journey, but I still have an end goal in mind and playing it out in my head, I think I can make it to that end goal while staying true to these turn in events.
What would you do in my position?