We are currently running AD2000 with Exchange 2000 and Exchange 2003.
(so we've already preformed a forest/Domain prep for E2K3).

Am I still required to perform the tasks in KB314649 (Mangled
Attributes) which are meant to resolve issues caused while performing
an upgrade to AD2003 while running a AD2000 and E2K environment?

(Does the E2K3 forest/domain prep correct this issue)