Apparantly, the mergress is not just a simple knob exchange (so to speak)
The engineer dismantles the razor, calibrates the mechanism for improved adjustment dial accuracy, enamels the dot and makes the numbers more legible. A number of people and B&B have said it changes the balance for the better (I have Ikon handles on my three-piece razors so like a heavier razor)
How true it is I don't know, but quality control on the standard Progress is poor and changing the blade the adjustment resets itself to all manner of bizzare settings. A Mergress will always go back together and be set to 1 following a razor change.
Too late now anyway... paid me monies await my choice.