You will need to reach out to the vendor of your nas or the firmware that it uses. We have no way of knowing what modifications they have made (and they are likely significant).
Standard OpenWrt does not (normally) boot a kernel with initramfs at all. The kernel is bare and matches the rootfs-- which is also replaced when upgrading.
GRUB does not need to be updated on an upgrade since the names and location of the new kernel and rootfs are exactly the same as the old ones.