Once I found my good old samsung netbook (little NC10 model, it was amazing laptop for my needs) and found that it has good old OpenBSD (I expected that).
But problem arise... it was really old 5.7 version. Time to update! But how? Official FAQ only supports close versions upgrades. Do it step by step? Too much time... Reinstall! But I don't remember, maybe something important already exists at home.
I quickly check documentation and can't find info how to preserve one of disklabel partitions while installing... but I found how it can be achieved.
IMPORTANT. I ALWAYS USE SEPARATED PARTITIONS FOR /HOME AND RECOMMEND IT TO YOU!
Boot your installation media, get to fdisk part
Choose O for using already existing OpenBSD disk partition.
Next page will be default auto disklabel...
Just choose [C]ustom layout, check with 'p' that you see your old disklabels, now use 'n' to set mount points for all labels that you WANT to format.
!!! DONT TOUCH LABEL(s) THAT YOU WANT TO SAVE DATA ON !!!
after setting all mount points for "new" labels use 'q' to save&exit and watch how only "mounted" labels got formated :)
Continue installation and don't forget to fix /mnt/etc/fstab for other mount points before reboot.