It is possible to update running system remotely with os-rebuild tool (this tool is similar to nixos-rebuild and accepts most of its parameters). You can build os-rebuild from vpsadminos/os directory by running:

make os-rebuild

which builds config.system.build.os-rebuild target. You can then use this tool to upgrade a QEMU instance with:

NIX_SSHOPTS="-p2222 -i~/.ssh/correctKey" ./result/os-rebuild/bin/os-rebuild switch \
  --build-host localhost \
  --target-host root@