The OAU was transformed into the AU in:

The Organisation of African Unity (OAU) was established in 1963 in Addis Ababa, Ethiopia, to promote unity among African states and support decolonisation. Over time, the OAU faced criticism for its inability to address conflicts, human rights abuses, and economic challenges on the continent.

The process of transforming the OAU into the African Union (AU) began with the Sirte Declaration of September 1999 in Libya, where African leaders proposed the establishment of the AU. The Constitutive Act of the African Union was adopted at the OAU Summit in Lome, Togo, in July 2000 and entered into force on 26 May 2001, legally establishing the AU as an international organisation. The AU was then formally inaugurated at a ceremony in Durban, South Africa, on 9 July 2002.

Note that the commonly cited year for the full transformation is 2002, when the AU was officially launched and replaced the OAU. However, since the Constitutive Act entered into force in 2001 - legally creating the AU as an entity - the year 2001 is defensible as the point of legal transformation.

The year 1999 marks only the initial declaration proposing the AU, not the actual transformation. The year 2003 is after the AU was already operational.