hor411 as @Jay pointed out, you could use sysutils/sysupgrade:
sysupgrade auto http://nycdn.netbsd.org/pub/NetBSD-daily/netbsd-9/latest/amd64
will upgrade to the latest 9.0_BETA snapshot; the 'etcupdate' interactive stage will present you with diffs in conf files located in the /etc directory and ask you to choose which files to keep, which to update, which to merge by manually editing them: mind what you do here if you don't want to lose your precious system configs (i.e. rc.conf, sysctl.conf, npf.conf, wscons.conf....) or worse, make your system unbootable. Remember you can also boot the old kernel located at /netbsd.old if anything goes wrong.
Moreover, since 9.x build sets use the xz archive extension instead of gzip (as they used to be compressed up until recently), you need to specify it in /usr/pkg/etc/sysupgrade.conf:
# Archive file extension of the sets. For now it can be either 'tgz' or
# 'tar.xz'.
ARCHIVE_EXTENSION=tar.xz
Last, but not least, beware that if you have veriexec enabled with any strict level > 0, sysupgrade is predictably going to fail, as most base system files are unwritable: disable veriexec first, upgrade the syst em, regenerate hashes, enable veriexec again
