hello rd1,
in theory, you may able to flash the images since the secureBoot did not enabled.
but I do not sure your system able to boot-up or not, due to I’ve never test this use-case before.
however, you may re-flash the board again to recover that.
device may not boot-up due to key mismatch.
the concept of Secureboot is to prevent execution of unauthorized code during boot process through chain-of-trust; those authenticates boot components (such as, Boot Configuration Table, bootloader binaries, and warmboot vector) were signed using private key.
PKC public key hash (stored in fuse) is validated by BootROM when it loads in BR-BCT (where public key is stored).
thanks