All depends where the parts come from, you might be able to find out on the DVLA web site, if you use all new parts and can provide recipts for everything, you regester it as a new car, but if you are doing a reshell and reusing everything from the donor car you can use that ID.

Its all done on a points system, but at the end of the day if you have an ID whos gonna now which car all the parts are from!