It is impossible to upgrade containers OS template to a newer version, the following message displayed:
[root@vps2 ~]# vzpkg upgrade 1851 vzpkg: Cannot find upgrade for debian-6.0-x86_64 OS template
Upgrade functionality was deprecated and no longer provided for OS Templates released after 2012.
In a ploop-based containers operating system can be upgraded manually from inside the container using native way, recommended by OS distribution authors. If you're running vzfs-based containers on PVCfL or PSBM you can migrate your containers to PCS, or upgrade the host to PCS to be able to upgrade container's OS.
Upgrade functionality exist in the earlier EZ OS Templates: KB117004