In fakt it's really simple.
First save a list of current installed packages and remove them all.
# pkg_info -m > packages.txt # pkg_delete -X
Than perform an upgrade as usual using a
architecture. Boot the upgraded system. Don't forget
in case you're mixing an regular upgrade and architecture switching.
I suppose an upgrade over extracting the tar's could work too, but
I didn't test it.
Finally install all previous installed packages for a new architecture.
# export PKG_PATH=ftp://ftp.openbsd.org/pub/OpenBSD/$(uname -r)/packages/$(uname -m) # pkg_add -z -l packages.txt
Boot your system again to be sure everything works as expected. And your are done.
There may be some extra work. For example if you are using
you shall dump all it's
rrd data sets prior migration
for i in *.rrd; do rrdtool dump $i > $i.xml; done
and restore them after
for i in *.xml; do rrdtool restore $i $(basename $i .xml); done