The geometry of a square grand is a flawed design inherently. If you think about the string placement, the distance from the front end of the keystick to the hammer varies with each keystick. This creates a different amount of leverage, which must be compensated for with weights added to the keystick and/or other adjustments to touch weight to get an evenly graded or balanced touch weight.
This is not difficult to accomplish on a clavichord, because the load on each key is the same. On a piano, each hammer is a little smaller as you move up the scale, creating an additional variable. If a square grand is rebuilt, the new hammers will be different, and there will be the difficult additional task of regulating the touch weight.