It’s possible to upgrade an Virtuozzo/OpenVZ CT/VPS from Debian 7 wheezy to Debian 8 jessie.
The only caveat is you might have issues with systemd depending on the hardware node (kernel version).
If you replace systemd with sysvinit you make sure the container will boot after upgrading.
cat << EOF > /etc/apt/preferences.d/local-pin-init Package: systemd-sysv Pin: release o=Debian Pin-Priority: -1 EOF
sed -i 's/wheezy/jessie/g' /etc/apt/sources.list
Also check /etc/apt/sources.list.d/*
apt-get install sysvinit-core sysvinit-utils
You can also do this after the upgrade, but make sure you do it before rebooting
This error can be ignored, it will work on reboot:
sysvinit: restarting...init: timeout opening/writing control channel /run/initctl .init: timeout opening/writing control channel /run/initctl
There will be messages about new versions of configuration files (e.g. /etc/ssh/sshd_config)