In production order, the episodes Takei missed were #9 of the season ("The Apple"), #13-21 inclusive ("The Trouble With Tribbles," "Bread and Circuses," "Journey to Babel," "A Private Little War," "The Gamesters of Triskelion," "Obsession," "The Immunity Syndrome," "A Piece of the Action," "By Any Other Name"), and #23 ("Patterns of Force").
I wonder, in how many of those episodes would Takei's character have had more than a (typically) minimal role in? I think he would have been wrong for the Chekov scenes in 'Apple and 'Tribbles, although he could have been added to the bar fight in 'Tribbles.
But I can see him replacing Walter Koenig and being a better choice in 'Gamesters. It would be interesting to see Sulu's reaction to Tamoon's line "If my Provider is pleased, we may even be selected for each other."
In 'Syndrome either actor could have done equally well in the Chekov scenes.
In the rest of the noted episodes, Takei participation would have been slight.