I recently upgraded an i386 machine from 1.5.4ALPHA to 1.6ish (along
netbsd-1-6) by just running ./build.sh with no arguments.  After
fixing passwd/group stuff, this ran to completion and I rebooted to
find myself with a 1.6 userland and a 1.5 kernel.  Fortunately from
this state I could build and install a 1.6 kernel and reboot,
and after fixing up etc everything was fine.

What is the right way to upgrade a machine from source to 1.6?
BUILDING and build.sh do not talk about building kernels.  Should I
build tools first, and then use the cross tools to build/install a 1.6
kernel, and then run the whole build.sh?  It would be nice if BUILDING
explained this, or if build.sh had a way to build and install a
particular kernel.  I presume that it does this for a 'release' build
into DESTDIR, but it didn't seem to for a regular build into /.
I think I'm looking for somthing like FreeBSD's "make KERNCONF=GENERIC world",
but I'm also open to hearing why that's a bad idea :-)