The problem with your painting analogy is a painting is static. Once Picasso lays his brushes down and signs it, it doesn't change. Oh, it has to be protected, but the painting doesn't get bigger.
Your car analogy doesn't work so well. There are companies that make modifications to cars and resell them under their brand name. Sure, they'll say it started as xxx car, but when they're done, it's something else entirely. (Sorry, I'm not into that kind of thing so I can't name specifics.)
It's not unusual for Kimura, for example, to take a tree that had been worked on by who knows who for years, and redesign it. Make it into something it had never been before. I have Japanese language magazines where they do exactly that! Sometimes, they'll take amazing trees, and completely restyle it. I have no idea why, I guess they had too much to drink! But when they're done, if you didn't know, there's no way you would have thought the final product and the starting tree were the one and the same.
Brian Van Fleet has an azalea that he calls his "Ben Oki" azalea. There's a thread on it here somewhere. The styling today shows no resemblance to the original style when Brian worked on it with Ben. Now Brian still calls it his Ben Oki tree because he wants to. But the styling is all BVF!