While in the laboratory, as the experiments were performed his body slowly became almost completely made of tentacle cells. If you look at other tentacle wielding humans like Itona and Kayano, their limbs don't become tentacles normally. This shows that even while still maintaining a mostly human appearance, most of his body was already tentacles just before his escape (Chapter 138)
When he made his escape, his body lost most of his human characteristics and took on the appearance of being full tentacles (still 138), explaining that the tentacles are influenced by the host's emotions. He underwent a grotesque transformation because he felt that he was giving up his humanity and becoming a monster.
Finally, his transformation to yellow occurred after he escaped in 140, underground. This lovable yellow form was because of his emotional change. He desired to be lovable, and he was happy (as demonstrated by the smile and the yellow coloration). He saw himself not as a master assassin human or a killing machine, but as a happy lovable teacher, so the tentacles shifted to accomodate these thoughts,
So, based on what we know about the tentacles, Korosensei's body has the capability to revert to his human appearing form. The tentacles have been shown to simulate a human body before, and they can take any form. However, this transformation is based on an emotional state. That means the user must feel or believe that they are the human they take the appearance of. Korosensei cannot do this because he no longer sees himself as the man that was the world's greatest assassin.