I get to wear my black belt from my last school, so I at least don't look like a newbie in class, but yeah I know what you mean. I actually know enough to test for Purple already, just need to practice and perfect one of the two new forms (the other new form for Purple I've already got looking super sharp as I've been practicing that + a black belt level form for tournament)
That's probably the most confusing part of my orange belt kenpo status / previously styles black belt status... the MP often just loves skipping a lot of the material I need for my next belt (since I've already learned it) and working on black belt level material instead since the adult class only has one other beginner in it, and the rest are all brown and black belts. I personally love getting to work on the advanced material, but sometimes I have to remind them that I need to practice my beginner stuff for my next test!
We have a guy who holds a black belt in TKD that just started in our Gumdo class. He wears his TKD uniform and his belt. So that probably helps him not feel like a complete noob. I know what you mean about learning stuff beyond your rank, Ive been allowed to join in on some of the more advanced stuff too. Its Fun!