An easy way to say would be when it's still 50% original, but I think an interesting way to look at it is that it becomes a whole new thing after every major change.

First it's his new car, then it becomes his new car with new tires, and then his car with new windshield wipers, and finally his old car with all new parts and some old ones. None of them are the same car.

I think in cases where it' a major rebuild, like turning a WW2 Minesweeper first into a ferry, and finally into Cousteau's research ship Calypso this outlook is more obvious. Are these ships all the same despite getting almost a full refit at each stage? I would say none of them are the same ship, but completely separate "things" with some old and some new parts.