Why did they replace the Oracle?

The Matrix (1999) and The Matrix Reloaded (2003) – She played the Oracle, however, she died during filming and was thus unable to portray her role in the third film. As a result, Mary Alice replaced her in The Matrix Revolutions and Enter the Matrix.

Why is the Oracle a different actress?

In reality, Mary Alice played the Oracle because Gloria Foster died of complications from diabetes before her role in Matrix Revolutions was shot.