Our partner has two possibilities to decide how the upgrade and downgrade functionality will work.
In MP, if "Use edition rank" is checked >
The edition ranking will be the deciding factor.
For example, changing the edition from 2nd position to 1st position it will consider as an upgrade. Even if the pricing of 1st edition is lower than the 2nd
If Use edition rank the option is unchecked >
the edition pricing will be the deciding factor for upgrade and downgrade.
For example, changing from low price to high price will be considered as an upgrade.
Manage > Marketplace > Settings > Billing Functionality > Subscriptions > Subscription changes: Use edition rank
As mentioned in the description of this checkbox, enabling it would mean that the subscription upgrade/downgrade will be based on rank.
Reference tickets :
Appdirect Documentation : https://help.appdirect.com/products/Default.htm#DevCenter/Prod-Add-Recurring-Edition.html%3FTocPath%3DProducts%7CEdit%2520editions%2520and%2520pricing%7CAdd%2520editions%7C_____3