Usually, the upgrades are seamless (freebsd-update(8)[0]); system files that change between releases are looped through with some help of sdiff(1)[1] with relatively few surprises jumping dot releases (12.3 -> 12.4).
However, major pkg upgrades also bring their fair share of config changes; and those major pkg upgrades usually happen during major version changes (11 -> 12). Can't remember how I did it, but at one point I had an ancient nextcloud running on a recent FreeBSD system where a pkg upgrade broke everything in a spectacular fashion.
The latest OpenBSD upgrade though (7.1 -> 7.2) did break an important base tool I used (dhclient(8)) by dumbing it down to a lesser version that is less capable (no auth) (but probably better written)[2], so YMMV.
> Changed dhclient(8) to defer to dhcpleased(8) by doing execve ifconfig and providing syslog warnings about deprecated options.
However, major pkg upgrades also bring their fair share of config changes; and those major pkg upgrades usually happen during major version changes (11 -> 12). Can't remember how I did it, but at one point I had an ancient nextcloud running on a recent FreeBSD system where a pkg upgrade broke everything in a spectacular fashion.
The latest OpenBSD upgrade though (7.1 -> 7.2) did break an important base tool I used (dhclient(8)) by dumbing it down to a lesser version that is less capable (no auth) (but probably better written)[2], so YMMV.
> Changed dhclient(8) to defer to dhcpleased(8) by doing execve ifconfig and providing syslog warnings about deprecated options.
[0] https://www.freebsd.org/cgi/man.cgi?query=freebsd-update&apr...
[1] https://www.freebsd.org/cgi/man.cgi?query=sdiff&apropos=0&se...
[2] https://www.openbsd.org/72.html, https://try.popho.be/securing-home2.html#and-openbsd-decided...