I'm not sure where the software is that keeps the boot loader from being unlocked, I would think that it's on the mobo. If one was to acquire a mobo with 9.16 on it, could it just be changed and unlock the boot loader?
Yes, you could do that, but I would think the mobo would already be in the phone, so why switch it out?
Also, just because it's on 9.16 doesn't guarantee that you can unlock it. Someone might have put the 9.20 update on and then downgraded. But, by that point, the exploit has been patched and you are hosed.
The ability to unlock the boot loader on non-dev RAZR HD phones was an exploit. It was not meant to be unlockable this way. It's also a physical change in the phone - a fuse is blown, which means the phone cannot be locked again, at least as far as we know.
The update that came after 9.16 patched this exploit. It updated the kernel, which means the ability to downgrade to get the exploit is gone. Yes, you can downgrade to a prior version, but the kernel will still be the patched version that fixes the exploit. The kernel itself will still be the latest update. That is why you must unlock the phone before applying any updates after 9.16.
The same is true if you don't root the phone before the latest 9.20/9.30 updates. The last exploit that was found before these updates was also patched.
Remember, being able to root and unlock this phone is done by security exploits found by developers. Each update fixes these exploits, which means new exploits must be found, but since this phone is not that new, most developers have moved onto newer phones. Finding the exploits is NOT an easy matter.
Google for how the exploit to unlock this phone was achieved. You'll see how much work it took for the developer to find this exploit. It's not easy and takes a lot of time.