I think they should keep the crown along with the other rewards, but change it to a simple 'Veteran' reward. Once the Veteran status has been unlocked, people could chose to opt in and become actual Mentors (perhaps make the icon a stylized M, much like the RP icon, something simple and easily recognizable).

That way, people flying the Mentor colors would truly be looking to help out new players while those who worked hard towards unlocking their Veteran rewards (and those who seek to unlock them in the future) can keep their goodies and continue to display their crowns.