Ssiscool

Answer: Season 3, when they received the Ninja powers, the glasses didn't work with the ninja hood and mask.

Answer: Presumably, because his eyesight has improved.

This answers the question of why, but not the original question of when.

Ssiscool

Answer: Probably to save on costs of filming the same scene. In the reused shots they have always morphed so you can't see the actors. They just have to pay them to do voice-overs rather than the entire scene.

Ssiscool