Finding packages affected by libtool13 -> libtool15 change?

[Chris Shenton]

Was updating a bunch of ports which suspiciously changed at once and
I'm stuck in a prerequisite hell.  I checked /usr/ports/UPDATING and
see: 

  20060223:
    AFFECTS: all ports using libtool as part of the build process
    ...
    devel/libtool13 no longer exists.  devel/libtool15 has been modified
    ...
    The main visible change will be that a large number of ports have
    had their plists modified (we now install libtool .la archives)
    and, as such, PORTREVISIONS have been bumped on ~2000 ports (roughly
    1/7th of the tree).

    Unfortunately, there is no simple upgrade path.  Short of removing
    all packages and reinstalling from scratch, the only other viable
    alternative would be careful use of portupgrade.  Given the large
    number of different ways in which libtool is used by other ports
    in the tree, this is a process that is likely to vary considerably
    from system to system, and as such, folks should be very mindful
    of running automatic updating software, such as portupgrade, making
    extensive use of the -n flag (and equivalents for other updaters)
    to see what will actually be rebuilt, before actually performing
    the upgrade.

Ouch. Is there a way to see which existing ports will need rebuilding,
and what order they should be rebuilt to get the dependencies right?

My trial-and-error approach to using portupgrade is very tedious and
not very successful.   I didn't see anything I thought I could use in
pkg_info and pkgdb. 

Thanks.