Well, I think it largely depends. If a practitioner is transitioning to a completely different art that is not similar ( Taekwondo to say, Kung fu for example ) Then they should start as white. They will be a skilled white belt and pull out strategies from different styles, but they are still new to art. But if you are transitioning from Taekwondo to say Karate, two arts that share numerous similarities, or even say goju ryu karate to kyukoshin karate, you should maintain some of your level.
I don't believe that you should keep your black belt in that case, but you shouldn't have to start all the way over. Instead, I believe you should start at around either half way or 3/4 the way there. You must learn all of the new corriculum of course, but you should still be put at a rank in which your skills are tested rather than having to trudge through months or years of doing things you already know how to do ( A kick in taekwondo is very similar if not the same as a kick in karate for example. The application is just different. Application doesn't take long to learn ).
It's a little different with sparring. If you already know how to spar, no matter what system you go into, you shouldn't have to wait months or years to gain the privileged to do it again, and you shouldn't have to face opponents much worse than you. In the case of my taekwondo to kung fu transition for example, the student, though a white belt, should still be able to spar with the black belts. They will have to learn any new rules and break any habits on their own, but they can do it. If they can't, the instructor can place them to students of similar skill, but with sparring, as long as you know how to spar from any style, you should be able to at LEAST keep your sparring rank.
I don't believe that you should keep your black belt in that case, but you shouldn't have to start all the way over. Instead, I believe you should start at around either half way or 3/4 the way there. You must learn all of the new corriculum of course, but you should still be put at a rank in which your skills are tested rather than having to trudge through months or years of doing things you already know how to do ( A kick in taekwondo is very similar if not the same as a kick in karate for example. The application is just different. Application doesn't take long to learn ).
It's a little different with sparring. If you already know how to spar, no matter what system you go into, you shouldn't have to wait months or years to gain the privileged to do it again, and you shouldn't have to face opponents much worse than you. In the case of my taekwondo to kung fu transition for example, the student, though a white belt, should still be able to spar with the black belts. They will have to learn any new rules and break any habits on their own, but they can do it. If they can't, the instructor can place them to students of similar skill, but with sparring, as long as you know how to spar from any style, you should be able to at LEAST keep your sparring rank.