How to upgrade or downgrade a Plesk 12.x/Onyx license between editions?
For Online Store customers
Licenses purchased in Plesk Online Store do not support upgrade or downgrade between editions.
When you purchase your license, in Plesk go to Tools & Settings > License Management and click Install Key to install the new license. There is no need to reinstall Plesk.
For Plesk Partners
Plesk Partner licenses can be upgraded in KA or Partner Central. If it cannot be done, contact sales representative.
When your license is upgraded, in Plesk go to Tools & Settings > License Management > Plesk License Key and click Retrieve Keys to update the license. There is no need to reinstall Plesk.
For any questions, contact Plesk Sales team.