How to encrypt an unencrypted file?

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Active Member II

How to encrypt an unencrypted file?

Like the situation in this post:

In this section:
,it says

Important: If Encrypted content store is enabled on an existing or upgraded Content Services installation, only new content will be encrypted but any existing content won’t be encrypted.

How can I encrypt an existing unencrypted content?
Is there a way to encrypt the legacy files?

Thanks in advance.