Bob O,
Due to some corruption in the core data on one system, recently I chose to upgrade by exporting the application data, and then re-importing the data into a fresh OpenACS 5.5. This was possible because there were few active packages, and each package was not critically interdependent on core services (such as bugtracker/workflow).
Bulk exporting/importing was an easier process for me than messing with a corrupt/developer core. File-storage, photo-album and some other packages already have bulk export/import in CVS head.
It was still necessary to have a working original system during the re-fresh to check against, but an advantage included being able to use fresh permissions strategies.
I had a couple of 5.1 systems running on aolserver 4.5.1 and PG 82 (outside of the compatibility matrix bounds). It required exporting the pg db and re-importing in pg82, and applying the info from the old nsd.tcl file to the latest config.tcl. The system was not accessible to users, so I don't know how robust it was, but it seemed to work. There were obvious templating issues that would have needed to be addressed.
I believe acs-core has some serious changes between using aolserver 3 and 4.51. maybe they are avoidable with a early core?
cheers,
Torben
