They're not interchangeable.
An addendum is an addition to something which already exists. You could therefore have an addendum to a schedule of events, if there was a change after the schedule was published.
Does that make sense?
November 11, 2017
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Two different words in my book.
Addendum = something you add at the end of a document
Schedule = a time schedule, or "to schedule an appointment", etc.
November 11, 2017
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