If you take a shovel, and replace the handle, the shaft and the head, will it still be the same shovel? That's a question without an answer, because it depends on the viewpoint - if you consider this particular shovel to be the sum of its parts, then it ceases being the same shovel the moment you replace the first part. But if you consider the shovel to be the thing you use to dig holes, you could say that it never ceases being the same shovel, because no matter how many parts you replace, it will still be the "thing you use to dig holes".
I don't think that really applies. The "purpose" is being a Linux distro. If it is changed to being 100% based off debian, it is still serving that purpose, but it would empirically false to call it a Ubuntu-based distro.
I think a better analogy would be if you take a shovel of brand A, then replace the handle, shaft, and head with those made by brand B, would it still be a brand A shovel? At what point does it cease being a brand A shovel and start being a brand B shovel?