Once e-3.0.4
is installed it is expected to update OTA with the System updater.
Edit (i case it is not clear what I first meant)
With this OTA update you could now lock
but
The install page advises to lock the Bootloader directly at install time. Users have previously struggled with this, example workaround Revert back to stock OS and relock bootloader - #4 by aibd
Edit Option #2
Use of the install ROM again will delete data but might be the chosen approach by many.