I think that the question originally asked was a good one, and it's
something that pkgsrc doesn't handle well.  Once, I tried the brute
force approach, but I lost too many configuration files.  I won't try
that again.  OTOH, I've done manual things to delete all packages, and
found stuff lying around that I didn't think should still have been
there.

Now's a good time to think hard about it, since as best I can tell a
lot of people are going to want to do that when they upgrade to 4.0,
since the change in gcc makes it hard to use old (pkgsrc) C++ libraries
with new builds, I believe.