According to wikipedia, Branagh was replaced by Tom Hulce at the "eleventh hour" for the role of Mozart.

Curry auditioned to play Mozart.

Mel Gibson auditioned to play Amadeus Mozart.

Hamill was considered for the lead role after playing Mozart on Broadway.

Amy Irving was considered for the part of Constanze Mozart.

Mick Jagger auditioned to play Antonio Salieri.

Elizabeth McGovern tested for the role of Mazart's wife Constanze.

Reynold was considered for the role of Antonio Salieri.

Meg Tilly had been cast as Constance, Mozart's wife. She injured her leg the day before shooting began and had to be replaced by Elizabeth Berridge.