according to SBS 61, the question about Sanji's look after the timeskip. Oda said Sanji is still the same and nothing changed about him except the goatee. His answer read as if Oda is not aware of the eye switching as he clearly didn't know what the question is pointing out to. He even chose "illustration mistake" which further shows his ignorance about what is going on.
D: There's something that ReAaaaaLy bothered me so I took a pen for the first time! And so, the ''question is Sanji's *BAM*! 2 years later.... IT'S JUST PLAIN OUT THERE!!! And why hide the *GGGGG*?? Please chouse out of my speculations below!
① Illustration mistake ② Image change ③ He's actually a twin named Yonji. P.N.Aoya
(*San in Sanji is 3 so this person changed it to Yon, 4.)
O: I apologize. I couldn't really hear because of the construction nearby, but Sanji hasn't changed at all, has he? From 2 years ago.. ah! Oh, the goatee, right? Looks like he let his goatee grow. It looks pretty wild and pirate-like and I like it a lot. What else.. he has always hidden an eye and... curled his brows.. didn't say anything new to Luffy and the co. when they were reunited... Nothing has changed! If I have to chose, I guess ① if you found anything.
Oda, although writing a detail oriented manga, is not very conscientious about details.. What he said about sharing with Luffy and co means that the only significant changes in the appearance of the crew is Franky's new looks and Nami's development (in Sanji's eyes).
Zoro's eyes was a mistake. Is Sanji's eye also a mistake which Oda didn't recognize?
Or is it that it's a casual and unimportant thing (The side you brush your hair to), that oda changed on purpose ornot, and doesn't care about.
I read on the wiki that he changed it upon conquering the okamas which sounded pretty cool, but pure speculation.