Question about replacing the motherboard to circumvent the bootloader

Hello fellow iFixonians,

Searched around the internet a bit, but haven't found a satisfactory answer to this mainly academic question.

Let's say I bought a Samsung Galaxy S5 from AT&T a while back. This particular model has a signed bootloader (along with Verizon and Sprint), which makes it essentially impossible to flash a custom ROM onto the device.

Now let's say I buy a T-Mobile S5 motherboard from our very own iFixit parts store, and replace the AT&T mobo in my phone with it. After dealing with ESN/MEID/IMEI numbers, unlocking carriers, etc.., would I then be able to proceed to flash a custom ROM on it, or are there other parts of the phone that would reject the transplanted silicon?

My gut says that this should be possible, but I'm not experienced enough in cell phone repair to know for sure. Thanks for any info you can give me!