No upgrading means you don't have to delete anything. All your existing data will stay where it is. (But backup first, of course.)
Uninstall of packages just hasn't seen as much attention as installing. Basically on the real systems most OpenACS developers work on, packages aren't deleted so its not a huge issue.
On experimental systems, I just reinstall. It doesn't take that long.
There _is_ a way to have all packages uninstall/delete themselves cleanly. Every package may implement an unmount/delete instance/uninstall package callback that runs custom code for each package when these events occur.
Not all packages have been updated to use these callbacks for uninstall type events yet. In general, as I said, most sites do not require deleting of data, so this area is neglected.
