In my TSD school, two girls (sisters) were black belts in TKD when they came in. They were talented girls. My instructor let them keep their black belts, but they had a year or something to learn the TSD stuff up to their rank, then they would officially test for the TSD black belt. There where a couple of others who did TSD (under BB rank) that also came to our school. My instructor started them at white belt to make sure they knew their stuff, then tested them for their old rank.
When I started TKD, they started me at white belt, but because of my previous experience, I'm advancing faster than the usual student. I have to learn their forms, but everything else is the same.
But, as far as an ego, I'm not the type to "rub it in their face" because I'm advancing fast. I'm more than willing to help anyone who needs help. And I agree that not just the seniors should get respect, but EVERYONE should.
As far as a TKD student joining a Kenpo school, I would start them at white, because it's a totally different style. If a TKD student moved and joined another TKD school, then I'd let them keep their rank and offically test them later.