Quote Originally Posted by Prussian to the Iron View Post
But I did just send an invasion force to capture a region with a Nanban trade port. But there has been no option to convert? I checked the clan management screens, but there is no conversion button. What's wrong?
Have you owned it for several turns? If not, wait and see. If so, the game must require the event to pop before conversion is possible, and that would make early conversion impossible.

No one seems to know for certain if this can be done, hence the "might" in my first post. Thanks to your efforts we should have an answer soon.