Changing the link length does change the roll center. On my Associated products, I created a CAD file that shows the roll center changes with different adjustments to link length and position at static ride height. For my AE stuff, adding a washer under the ball stud in the rear changes the roll center more than changing the link length by 1 hole on the hub carrier. The thing is, with the change in link length, you also get a different amount of camber gain on suspension movement and a different rate of roll center change (instant center) over the movement of the suspension.