How Parallels Plesk Panel inside Parallels Virtuozzo Container registered in POA can be upgraded to the latest version?
The procedure is described in the guides Upgrading Parallels Plesk Panel on Customer's VPSs for POA/PBA-E 5.5.x and Upgrading Parallels Plesk Panel in POA for POA/PBA-E 5.4.x.
They contain complete procedure of transferring customers from old Parallels Plesk Panel version to a new one, including service template and plan switching in POA and PBA-E.
Note: In case production environment is involved, we recommend that you test the whole scenario on your installation using test service plans, templates and subscriptions first.
As an alternative to upgrade, you can use data transfer procedure. It can be performed by the container owner. Migration instruction can be found at the article How to transfer Plesk data between PVC containers registered in POA.