We have seen it in One Piece world dude... if you still doesn't want to approve it, then it's fine but facts are facts.
For example, Whitebeard: He always had a muscular and strong body since he was a teen/adult man BUT he also had long and blonde hair while he was relative young then... plofff, he got bald!
It's ok dude, Denjiro is Koshiro xDD
At least it makes more sense then Scopper, which is strangely someone in every damn arc of one Piece. Everytime someone sees Scopper anywhere.
And then the appearance changes is also proven inside the Manga, if we take BM, or Hyoguro... so your "Facts" are no Facts, just something you want to see there.
And yes, Denjiro = Koushirou is at least a theory based on Facts, based on Panels and based on a senseful plot. Nothing pulled out of the air with nothing in Hand, besides completely common glasses.