I haven't tried it, but hit the ~ button to launch the control console

remove_trait "Firstname Lastname" Factionheir 1
give_trait "Firstname Lastname" Factionheir 1

You have to have the correct original name, which can be difficult its its changed with the addition of Prince or other cog-name. If the character has a location as part of the name "Firstname of_Location" with the underscore works