I agree with changing all of them. Whats sucks is not knowing just what caused the problem and if you replace all these parts and it takes them out again. I dont know if your looking for more suggestions but I'll throw some out there anyways. First did you check the other mosfets? Q600,Q601 and Q602.I would also recommend checking diodes D609 and D610. I would also check caps C600,C601, C602, C603,C607, C608, C610 and C611 for shorts to gnd. Then I would disconnect CN89 and CN90 to the flyback and check my 200v at TP600 on the PA board. Then if it was me I would replace the parts but leave the flyback disconnected and run if for awhile to make sure the circuit holds, without blowing again. If it does I would then shut it down and reconnect the flyback with CN89 and CN90 and see how it does. If it then blows, you at least have an idea what is causing it.