Again I don't think we are fundamentally disagreeing so I will leave this thread because I am communicating poorly and confusing matters.
I think by arbitrarily all I mean is that I don't think they want us to upgrade (ie change a difficulty die to a challenge die) after the GM has decided on the initial difficulty of the task. They go out of their way to talk about limiting upgrading to talents and destiny points. Am I saying you can never do it? No, just like anything else in this type of game never say never.
Anyway this discussion is not really relevant to the original question so I will now out of this thread and go back to lurking.