To elaborate a little on:
@ermal:
You might get interface mismatch otherwise should just work.
If you keep everything else the same and swap the motherboard the next boot MIGHT report an interface mismatch because the onboard NIC (if any) might not be the same as the onboard NIC on your existing motherboard. This is relatively easy to correct at boot time on the console.
If you are looking for someone with experience to report then you will have to give more details (at least brand and model) on the motherboard you are planning to swap in.
The FreeBSD documentation (man pages and supported hardware list) doesn't appear to list that particular model but I've seen a wide variety of Realtek chips listed in the pfSense forums as working.