The double chairs are in good shape. Many have been replaced recently.
I learned last year while the chairs do have value, most of the value lies in the grips. Good chairs with bad grips aren't worth much and good grips on bad chairs do have value.
I also learned recently that the double and triple have different haul rope diameters. The triple chairs from Windham fit the double, not the current triple.
Personally I don't see any reason why you'd need to increase the uphill capacity of the double. I've never seen a line on the double outside of a line for first chair on a powder day.
i believe the thinking is triples have more room and skiers like it.