Removing ports based on library dependency

Hi,

Apologies if this would seem to be the wrong list for this post.

I want to upgrade one of my FreeBSD boxes running 6.0 to CURRENT. I
know that this involves rebuilding ports so they link against
libc.so.7 and all associated libraries.

I guess the small problem is that I really only want to upgrade
certain ports, since the machine has a ton of stuff I really would
like to get rid of. I can set those individual port upgrades off
myself with portmaster. However, I would then want to delete all the
ports that I didn't upgrade...and presumably that would mean checking
what was still compiled against libc.so.6

The problem is, I have NO idea what the incantation would be to get a
list of each installed  port's dependencies...but once i have that, I
can probably do a pkg_delete on it...and with CURRENT I could probably
clean up with delete-old-libs target?

I would greatly appreciate if somebody could advise me how to go about
this....and although I know it would probably be more efficient to
reinstall the machine from scratch, I'm trying to avoid the data
shifting and inability to use the system that alternative causes.

One extra little point, is symbol versioning reobust enough to be
enabled for the entire system? Are there any bebefits/drawbacks to
that approach?

Thanks in advance, Alex J Burke.