See 'make replace'.  It does the build, and if successful replace, but
a) only for a single package, and b) since the depending packages
aren't reconfigured, they might not work if there is an ABI change
(e.g., shlib version).

I have long thought about writing something that flags packages that
have had dependencies 'make replaced', and runs 'make replace' on
those, so one can cascade this through, ending up with a consistent
system, with no massively broken period, but some risk of lossage
during the intermediate time.

But now I use pkg_comp and pkg_chk (in pkgsrc/pkgtools) to build all
my packages in a chroot, and then 'pkg_delete -r \*' and pkg_add all
the new ones.  It's a bit of work to set up, but not much, and I've
had very little grief with this approach.

-- 
        Greg Troxel <gdt@ir.bbn.com>