Which of these two sentences is correct and why:

The schedule has changed.

OR:

The schedule has been changed.

A:)

They are both correct.

This is not a question of tense. It is a question of passive and active voice.

'The schedule has changed' is an active sentence; i.e. the subject (schedule) performs the verb (has). The word changed is an adjective and is an object complement.

'The schedule has been changed' is passive; i.e. the subject is having the action performed upon it.

Good question. Happy to be corrected on my answer.