These days the device tree (which is most of what the BSP changes are when using an alternate carrier board, but I have no experience with the J120) requires signing and then writing to a partition. If the new signed tree fits in that partition, then in theory you could use dd to write it if the system runs on any carrier board.
On the other hand there is a lot of risk with this and it won’t be simple. You’d be advised to clone the rootfs before you make any kind of flash or dd update (then if it overwrites the rootfs it will overwrite with what the content already is…essentially not changing anything).
In theory you can flash just the device tree and not change the rest of the system, but I am hesitant to give steps because I have not flashed just device tree (I always have a clone in place “just in case”).
If you do want to know more about cloning the rootfs, then see:
In that above URL you’ll see notes for a patch to flash.sh, but that is only for earlier releases. No patch is needed for R28.2.1 cloning.