How to upgrade from one OpenBSD version to another (intern docu)
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sync the template directory with the files found in the vanilla OpenBSD:

  the following files can just be copied, they don't have local adaptions:

  cp /etc/group template/etc/group
  cp /etc/moduli template/etc/moduli
  cp /etc/pf.os template/etc/pf.os
  cp /etc/login.conf template/etc/login.conf
  cp /etc/protocols template/etc/protocols
  cp /etc/services template/etc/services
  cp /etc/gettytab template/etc/gettytab
  cp /etc/master.passwd template/etc/master.passwd
  cp /etc/ssh/sshd_config template/etc/ssh/sshd_config
  
  the following files are customized, so adapt the by hand if concepts
  change upstream
  
  template/etc/boot.conf
  template/etc/resolv.conf
  template/etc/syslog.conf
  template/etc/fstab
  template/etc/newsyslog.conf
  template/etc/tabs/root
  template/etc/passwd
    cp /etc/passwd template/etc/passwd
    adapt to keep the root entry and set it to the password you like
  template/etc/rc
  template/etc/ttys
    cp /etc/ttys template/etc/ttys
    keep the adapted tty00 entry instead of the vanilla one
    
sync config directories

  cp /etc/network config/firewall-test/network
  for other configs sync it and keep your local networks there
  cp /etc/network confug/xxx/network
  adapt and add your local network(s) at the end
  /etc/joe copy and adapt, keep it as minimal as you like
  
those configs are really yours

  config/xxx/dhclient.conf
  config/xxx/rc.services  
  config/xxx/nsd-xxx
  config/xxx/relayd.conf
 
build the image

  ./build.sh xxx xxx
  
mount it and test it in the chroot

  vnconfig vnd0 xxx.img
  mount /dev/vnd0a /mnt/fw
  chroot /mnt/fw

you cannot test the router and the config itself, but at least
you can test in the chroot whether all binaries have their corresponding
shared libraries and do actually start.