When you upgrade from 1.1.8 to 1.2.0, and try to commit something, you'll get this error:

vyos@vyos# commit
Couldn't open /opt/vyatta/etc/config/archive/commits - Permission denied at /opt/vyatta/share/perl5/Vyatta/ConfigMgmt.pm line 108.
run-parts: /etc/commit/post-hooks.d/01vyatta-commit-revs.pl exited with return code 13

The root cause is that the GID of the vyattacfg group has changes. VyOS 1.1.8 used GID 104, but in Debian Jessie, 104 belongs to systemd-network, so when vyattacfg group is created, it gets a different id.