non-destructive ports/packages update

[Perry Hutchison]

I'm looking for a way to move everything connected with ports and
packages aside, so that I can start fresh but with the ability to
easily roll it back when things go badly (as they surely will).

I have in mind to something like this:

# cd /usr
# mkdir old
# mv ports local old
# mkdir ports local
# cd /var/db
# mkdir old
# mv ports pkg old
# mkdir ports pkg

Is there anything else that needs to be saved before fetching a
new ports tree and starting to build things (or install prebuilt
packages)?

This is on 8.1, although I suspect the answer does not depend on
the OS version.  And yes, there's plenty of space in /usr and /var.