When I took TKD, the instructor didn't care what belt we were in what style (even if it was TKD elsewhere), he made us all start at white belts...I had a blue in TKD at that time, so he kinda pissed me off.
My kenpo instructor honors all belts from all styles. If you're a black belt in TKD and want to learn kenpo, you still wear your black belt if you want to, and are given all honors and respect normally accorded to black belts. However, he sticks your tail in the white belt class and only advance to the next class when he thinks you're ready. Of course, if you actually wanted to start as white belt, he'd let ya, and his testing fees are only like $25 per rank (black is more, dunno how much...I'm only brown). so money isn't really that much of an issue if you want to start over and earn kenpo ranks too.
Personally, I like starting over when I switch styles, because it lets me better measure my progress...don't think I'll be switching again, tho. Kenpo and I get along well.
If I walk into a school and the instructor tells me he charges a hundred bucks for a belt test, I walk right back out the door.