If you want to downgrade a license key, you need to use the downgradeKey call with the same identifier that you used for the upgradeKey call.
Important: The common mistake is to downgrade a key by providing the key with to and from features interchanged to upgradeKey.
For example, to downgrade PLESK_100_DOMAINS_TO_PLESK_HOSTING_SUITE someone may construct the new key PLESK_PLESK_HOSTING_SUITE_TO_100_DOMAINS and feed it to upgradeKey. However, this will not downgrade the key. For the proper downgrade, call the downgradeKey with the original key.
Generally, if you construct identifiers on the fly and get errors, please refer to the Supported Identifiers section to check whether the identifiers actually exist.