I have a set of Shimano 600 brakes on a Centurion that I want to equip with Kool Stop pads... and the holders are like the Mafac style, except that the pad is enclosed on all 4 sides by metal flanges. The back isn't open to slide a pad in or out like those pictured above.

In the past I've just replaced the whole holder and all with a new style unit, but I want to keep this bike looking more original. How do I replace the pads? Just shove the old pads out and brute force the new ones in?

Or is there a trick that I'm missing?