I can`t find a setting to do this
Best answer by Transparent
A Forum User-ID change can be done by one of the Moderators, such as
Once they make the change, all your existing messages to the Forum will automatically change to display the new ID. So there will be no public record of what your ID used to be.
I suggest you click on the two Moderator names I've just mentioned, and send each a Private Message by clicking the purple Send Message button on their Profile Page. Choose a memorable non-descript name, such as I have, and they will check to make sure it's not already taken.