It occurred to me that Holger Weiss wrote in gmane.os.netbsd.general:
> * Huub <huubvanniekerk@gmail.com> [2006-04-16 20:55]:
>> Additional question (don't really understand this from userguide): after
>> cvs, do I have to "make install clean" or some kind of update-command
>> for upgrading software?

[...]
>| You can then use make update to update the package on your system and
>| rebuild any dependencies.
>
> [ http://www.netbsd.org/Documentation/software/packages.html#pkg_versions ]
>
> You could also have a look at pkgtools/pkg_chk.

Alternatively, you might want to look at wip/pkgmanager. Operation is pretty
straightforward: You tell it what packages you're interested in (Say, 
meta-pkgs/kde3), and it will find out what currently installed software is
outdated, what software is (no longer) required, or what software has an
updated dependency. Based on that, it will figure out the most
efficient build order.

It's a bit like lintpkgsrc, pkgdepgraph and a few others rolled into one. The
big difference is that it will try to do replacements in place, instead of
removing all packages. This usually results in less downtime.