When subscribing to Salesforce.com, Environment licenses (i.e. Editions) can change, but they can only go up in edition, not down. So for example in the case of Sales Cloud you can start with Group Edition, upgrade to Professional Edition, upgrade to Enterprise Edition and finally upgrade to Performance Edition. You cannot go the opposite way. So once you reach Enterprise Edition, you can only upgrade to Performance Edition and not downgrade to Professional or Group Edition.
If at all you want to downgrade the workaround is to export all data in CSV files, subscribe to the new edition of Salesforce.com, import the data from CSV files and cancel current edition license. Depending on the complexity of the application and the amount of data – this can take substantial time and effort.