Question about Outlook client config

I have several thousand Outlook 2003 clients that currently have their
"Deliver new e-mail to the following location" set to a local PST. I need a
reliable way to force all of these system to deliver to their Exchange
mailbox without losing all of the other settings within their Mail profile.
I have tried to use a PRF, but the only way that I can get the PRF to force
the change from PST to mailbox is to use the "Overwrite existing Exchange
settings if an Exchange connection exists," but this seems to "overwrite" too
many other options to be a viable options. Is there a registry key or a GPO
setting that can force this change?

Thanks for the help!