Hi breakfastburrito,
I don't see the need to declare a PAN if the destination weather is above landing minima. That's a bit of overkill.
I'm afraid i can't add much from the Australian perspective, but fwiw, in my company we differentiate between flight planning and in flight considerations. This may include nominating a new destination and/or alternate in the air, but the primary motivation is a safe conclusion of the flight based on actual conditions.
The academic nature of this discussion is probably appropriate on the ground but not in the air. As in the Qantas A330 incident, if the **** really hits the fan and the weather craps out, you just do whatever you can.
Cheers.
edit: typo