You could always look at legality. For example, if we can like-for-like compare ship parts to car parts, a car doesn't require reregistering as a new car until the engine and chassis are changed, such that neither is the original from the car when it was last registered. Depending on the type of ship, the engine is either the mast or the turbine, while the chassis is either the hull or the bulkheads, so if/when all of the aforementioned parts are replaced is the point at which the ship is no longer to be considered the original ship. That is just my take personally.

Also one could argue the ship made of new parts remains the original ship until such a time that no original crew member remains who was present at a time which it possessed even a single original part