Can the fireplace refractory panels be replaced with firebicks

You are talking about a prefab fireplace like a Majestic, right? If you ask the manufacturer they will say no and probably have a good reason. I would not do this job on someone's home since I could be creating an unsafe situation. There will be people who will tell you it is fine and to go ahead. It is their opinion but they probably do not work in the trades so be cautious who you listen to.