why did he change his last name?

This message has been deleted by the poster

I don't know for sure, but I tend to think that what motivated it was the fact that his sister, Melinda Kinnaman, was already an established actress when he first started out. Among other things, she had played an important role in the 1985 Lasse Hallström movie "Mitt liv som hund" ("My Life as a Dog"), which was released in the U.S. in 1987, won a Golden Globe for "Best Foreign Picture" in 1988, and was nominated for two Oscars the same year in the "Best Direction" and "Best Adapted Screenplay" categories. A bit of name recognition doesn't hurt.

Why did Martin and Charlie Sheen change their name? Why did Elton John change his name?

I read he was born with the surname Nordstrom but isn't Kinnaman his father's surname?

Maybe there was already a Joel Nordstrom registered with the Screen Actors guild.