For paid accounts, Gliffy will never change the status of any private diagrams. Specific information regarding this is as follows:
Trial Accounts - On trial accounts, documents are created as chosen by the creator, as permissions within Trial accounts are equal to that of paid accounts.
Free Accounts - Diagrams created on free accounts are publicly viewable by default.
Paid Accounts - Diagram privacy is determined by
When a paid account lapses, administrators of the account are presented with an option to either re-upgrade, or convert their account to a free, basic account. If the administrator chooses to re-upgrade, diagram privacy settings will be retained. If the administrator chooses to convert the account to a free account, they confirm and agree to their diagrams being transitioned to public diagrams.
You can check diagram privacy by reviewing the icon as shown below: