It was never established that Sulu was Japanese. In fact, the TOS Writer's Bible suggests that Sulu had mixed southeast Asian ancestry, including the Philippines. Sulu isn't a (current) Japanese surname, but the Sulu Sea is in the southwestern Philippines. But for all we know, Sulu really is Korean, and now is the first time that he is being played by an actor with the same ethnicity.
Sulu isn't even remotely an ethnic Korean name, but if the argument is that he was born in Korea, rather than San Francisco, then we could just as easily redefine Scotty as actually being German or Chekhov being from Sweden. After all, Germans, Scots, Russians, and Swedes must be interchangeable because they all have pale skin and identical features.
No -- I'm not trying to say that. You mistake (or exaggerate) my intent.
All we know about Hikaru Sulu's background is that he was born in San Francisco. He didn't even get the name "Hikaru" until TUC in 1991.
People
assume that he is 100% Japanese, but that's been established nowhere, even contradicted by the TOS Writers' Bible. Should Hikaru Sulu be Japanese because George Takei is? Maybe. Maybe not.
My last sentence was a bit of joke. If "Sulu" is neither a Japanese nor a Korean name, that's no help is establishing he is ethnically either. So perhaps Sulu "really" is Korean, and George Takei was the actor who was miscast, not John Cho.
Barring any evidence to the contrary, I'll believe what the TOS Writers' Bible establishes: Sulu is of mixed SE Asian ancestry.
P.S. - Your "Scotty" argument is pretty weak since James Doohan is Canadian and of Irish ancestry.