Grafting: Grafting branches or buds of one flower to another. Metaphor is to deceive others by secretly changing people or things.

Stealing the column means: it is a metaphor for playing tricks in the dark, confusing the fake with the real, and substituting the inferior for the superior.

According to the meaning of the question, they changed the original BMW Z4 into a Mercedes roadster, which means to cheat others by changing things.