pkgng: How do I upgrade a single package?

[Darren Pilgrim]

Now that we have a repo, I want to start using binary packages for the 
numerous dependencies I have to install on pretty much every system: 
autotools, ca_root_nss, cmake-modules, gettext, glib, libtool, m4, etc.

But `pkg upgrade` only asks me to upgrade everything at once.  This 
absolutely won't work on any of my systems because I need non-default 
options on a few ports and therefore must compile those.  But I don't 
want to compile everything from source.  For example, of the 193 ports 
installed my home server, I set non-default options on only 15 of them. 
  Everything else I can install as a binary package and save myself a 
HUGE amount of time.

How do update individual ports from pkgng packages?  Can I just install 
the new version over top of the old one?