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You can enable the password at any time, all data will be left intact. I’ve both set and unset the password several times without any problems.
However, setting the password doesn’t encrypt your data, it has been encrypted since day one. Setting the password simply encrypts the keys used to decrypt your data.
In my experience you also have to enter the password if you start the computer from hibernation, but not from sleep.
That's the answer I had hoped for!