I think I dodged a bullet!

I copied the wphost.db and ngwguard.db from a backup prior to the user/resource move on the old server, and brought up the PO on the old server. I then logged in with the GW client and connected to the old PO using cli options:
C:\Novell\GroupWise\grpwise.exe /@U-gwresourceowner-ccpo1 /ipa- /ipp:1678

I then was able to proxy into the resource and see all the mail. I set the archive path to a network location and I then archived all the email.

I then logged into the new PO as the resource, pointing the archive path to where I archived everything, and then unarchived all the items, effectively transferring all the emails from the old PO into the new PO.