Is it possible to upgrade or downgrade Plesk 12/Plesk Onyx license between editions?
Licenses purchased in Plesk Online Store do not support upgrade or downgrade between editions .
If you would like to switch your license from one edition to another, please cancel your current license (as described in How to cancel a license purchased from Plesk Online Store ) and purchase a new license for the required edition in our Online Store:
Plesk Partner licenses can be upgraded in KA or Partner Central. If it cannot be done, contact sales representative using the link below. Additional information can be found in Plesk Documentation.
If you have any questions, contact Plesk Sales team :