Perhaps I'm missing something here in respect to Metro's posting re his scenario 1 & 2?
Unless I read you wrong Metro, you're suggesting that you would depart A for C, with the legally required fuel, and then when over or abeam C would determine if you had enough fuel to go to B (which is where you really are supposed to be going) and then change your flight plan and go there!
OR (scenario 2)
When over or abeam C if you don't have sufficient fuel to go to B (which is where you are really supposed to be going) then you simply land at C much to the surprise of the cretins down the back!
Have I got that right? If so, we will have to resit flight planning!
If I have it wrong, then I apologise.
You can of course depart A for B and replan over or abeam C, but not the other way around! I suspect, however, that that is what you meant!