I believe it just needs to go through a conformity check looking over logs and verifying all required mx has been done, fire extinguishers are onboard, seats are fireblocked, all placards are in place...Which would be done already if already on a certificate, but some people are more thorough than others. I know of a Gulfstream that passed one check at one company, but the next operator found 80,000 dollars of mx that needed to be done.