I would think that when a paper is re-formulated, that there might be a slightly different spectral sensitivity. I.E. example MG3/MG4 it would be important to use the newest filters matched to the paper. The difference might be slight, but using the correct filter values will eliminate another variable in the process. Perhaps Simon may comment on this. I know that analyzing filter values from MG2 to MG4 there is a slight difference. That is without taking fading into account.