As per the information provided on the support page, once you enable the settings, it will not affect the messages previously sent or received in chat. All the users will be able to turn disappearing messages on and off for individual chats while in a group chat, only the admins will be able to use the feature.
WhatsApp Disappearing Messages: Soon to Hit Your Devices
If a user doesn’t open WhatsApp in the seven day period, the message will disappear. However, the preview of the message might still be displayed in notifications until WhatsApp is opened. When you reply to a message, the initial message is quoted. If you reply to a disappearing message, the quoted text might remain in the chat after seven days. If a disappearing message is forwarded to a chat with disappearing messages off, the message won’t disappear in the forwarded chat. If a user creates a backup before a message disappears, the disappearing message will be included in the backup. Disappearing messages will be deleted when a user restores from a backup.
Note: Only use disappearing messages with trusted individuals. For example, it’s possible for someone to:
Forward or take a screenshot of a disappearing message and save it before it disappears. Copy and save content from the disappearing message before it disappears. Take a photo of a disappearing message with a camera or other device before it disappears.
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