I think with this new system its going to be a case of nearly every flight/route being different and people will have to do their own due diligence. I seldom use miles for upgrades and when I do its always been on B fares so this would affect me little, that said I recently noticed some weird pricing to some EU cities for example my upcoming LAX-LHR-MUC is an M fare and was only 1200, which I was pretty surprised by how cheap that was for M but I still used SWUs anyways as I have them so why waste the miles.

I suspect we will have quite a few threads of this nature in the coming months and again I think there's not going to be a single "right" answer and people will have to check all the available fares, the cost differences and their own needs (ie do you need/can you justify a less restrictive fare? == lower copay) and price sensitivity.