Comment by volkadav

The upgrade from 7.9 to 8.0 went completely smooth on a bastion host I have running with openbsd; from start to finish it took about ten minutes. Kudos to the team!

https://www.openbsd.org/faq/upgrade80.html

tl;dr - doas sysupgrade, system updates itself and reboots, doas sysmerge, doas pkg_add -u.