No, that's not true, but the answer wouldn't depend on it being "the same person". By premise, you have a person, then you lose that person, then you get a new person that just happens to inhabit the same physical body. It's very weird--never actually happens--but there we go.

The thing is, even if it's not a moral issue due to personhood, it might be relevant that the individual could help or harm others. All else being equal, humans are a social and cooperative species, and generally we are more help to each other than harm. That the guy was homeless raises some doubts there, and if he was going to be miserable then it could be a cruelty to consign him to that fate, but we'd really need to know more. The default stance would be: though there's no intrinsic value without personhood, there is still potential instrumental value, and thus the body should probably be preserved.

Still, in terms of moral outrage, for me it is more along the lines of you finding a bridge that might be useful someday but nobody's using it now or even remembers it, and you blow it up for fun. Flippant destructiveness of items of value is wrong, just not anywhere near as wrong as murdering people.

But, again, it's a weird situation; the closest real situations are dominated by uncertainty.