On another thread somewhere....
someone posted that they have students that come in with outside rank wear a white belt with whatever rank they hold as a stripe through the center (so if you were a red belt before, you wear a white belt with a red stripe through it, and then a yellow with a red stripe, etc). That way other students know that this student has trained before (so they aren't discouraged when that student moves faster or kills them in sparring) but also allows the instructor to be able to see what the student knows and needs to learn in their system (which is what I believe the whole point of a belt system is... so the instructor and other students can immediately know what the student knows, needs to learn, and if they can be looked to for an example). I really like this system and plan to suggest it to my instructor the next time someone transfers in... unless of course some finds some glaring problems with it!