tienes que ser s-off para cambiar el hboot.

aquí te dejo los pasos para cambiarlo.
1) Nandroid backup in recovery.
2) Verify the MD5SUM of the file you downloaded against the one in the table.
3) Flash HBOOT with your phone in fastboot mode (Back+POWER) -> 'fastboot flash hboot bravo_alphaspl.img' (change into the correct filename for the HBOOT you downloaded)
4) 'fastboot reboot-bootloader'
5) 'fastboot erase cache'
6) Boot recovery, wipe everything and restore.

Mas info: aquí