Finaly,
only if the android security update in the previously installed Operating System was older
than the android security update included in the /e/OS build you want to install
you will be abble to relock the bootloader according to instructions at the bottom of the device install page (only possible with Official version)
check bootloader status by running :
./fastboot oem device-info
./fastboot flashing get_unlock_ability
if the return is 0
DON’T TRY TO RELOCK
risk is to brick your device (wait untill next /e/OS update)
if the return is 1
→ you can relock