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.

1)

cat << EOF > /etc/apt/preferences.d/local-pin-init
Package: systemd-sysv
Pin: release o=Debian
Pin-Priority: -1
EOF

2)
sed -i 's/wheezy/jessie/g' /etc/apt/sources.list
Also check /etc/apt/sources.list.d/*
apt-get update

3)
Install sysvinit:
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

4)
apt-get upgrade
apt-get dist-upgrade


There will be messages about new versions of configuration files (e.g. /etc/ssh/sshd_config)

reboot