i believe that there's still no software method to upgrade the bootloader. btw people with latest bootloader doesn't have advantage of unlocking it with latest baseband, but there are some reason why people insisting to be on BL4.6. this version can easily downgraded to a custom BL3.9 that is targeted as a super unlock (if it is made available). if u r on 3.9 now u can't go back to 4.6 by software means.