I think that Sadly the Captain Sulu boat has sailed.
I think he'd be fine now as Admiral Sulu, like he voices the new Federation 'book'. Koenig, with all due respect does look quite a bit older in the Renegades trailer.
Same with the other TOS castmembers bar Nimoy or Nichols I feel.
Walter just looks older cause he isn't wearing the wig in Renegades. Put the wig on, add some rose to the cheeks, and he will look just fine. Movie magic
As for Sulu, I would be happy to see him promoted to Admiral, while still serving as Captain of the ship. We saw Admiral Riker in command of the Enterprise, so no reason why Sulu couldn't be an Admiral. I just saw George in an Old Navy
commercial last night, and he still looks fine to play the part.