The clue is in the question
mkinitrd will not let you overwrite an existing ramdisk image (such as the default initrd file installed in /boot) unless you use "-f" option.
Since the file is critical to the boot process, personally I make a copy of the old initrd file first, in case the new one doesn't work. You can always select a different initrd by interrupting the grub boot sequence and editing the initrd option.