It is not sufficient to simply upgrade front-end servers to Exchange 2003 for users to get the new interface.

You must upgrade back-end servers to Exchange 2003 as well

Interface matrix:

Ex2000 FE + Ex2000 BE = Ex2000 OWA

Ex2003 FE + Ex2000 BE = Ex2000 OWA

Ex2000 FE + Ex2003 BE = Not supported (AG protected)

Ex2003 FE + Ex2003 BE = Ex2003 OWA

Ability to Reply and Forward to Messages and Posts in Public Folders is only enabled when the client is using a front-end server. Forms-based authentication (FBA) is functional for deployments where the FE is Exchange 2003, but the mailbox is still on Exchange 2000. However, session timeouts are handled much better if the BE are also Exchange 2003