The changed size of time_t means the format of binaries changed. One possible consequence: login doesn't work.
If you can't log in after upgrade and reboot, you've trashed something. Most likely the password database (/etc/pwd.db) needs to be regenerated. The following helped me regain access to my laptop after the time_t bump, jumping from snapshot to snapshot:
Boot from the newer bsd.rd, do the upgrade (at least until you have the file systems mounted), then before the reboot
# pwd_mkdb -d /etc /mnt/etc/master.passwd
And after reading the following -current page,
# cp /dev/null /mnt/var/log/lastlog # cp /dev/null /mnt/var/log/wtmp
And life was better. I was able to log in.